Nitrification not beginning in new tank, and PH/hardness mystery

[nick elton]

Hi all,

My first post here. Until a few weeks ago I had little interest in aquariums or fishkeeping, but my son caught me by surprise by asking for a fish tank for his fifth birthday, a few days before the big day.

I’m not one to rush into something without research and I found out pretty quickly that there would be no way he’d have fish on his birthday. As a compromise I gave him a 60 litre tank on his birthday and we went to the fish shop to look at fish which might be suitable.

One of the reasons that I’d never been interested in aquariums before was because of the experience most people my age would have had growing up, of seeing some very large and sad goldfish swimming around with a plastic castle and some dirty gravel in a small bowl. I am an animal lover, and it would certainly make me sad seeing a site like this in my house, and even though this was a gift for my son, I will have to look at it every day and need something that makes me happy as well as him

Fast forward, and I have a 60 litre tank with a Fluval U2 internal filter. I have purchased the following substrate and hardscaping from Aquarium Gardens and set it up in the tank:

• ADA Power Sand Advance as base layer
• Tropica Aquarium Soil, capped in some areas with ADA Aqua Gravel
• A few pieces of Elderly Stone and some pieces of wood

I plan to plant the tank reasonably heavily, to ensure that the tank is something that I enjoy seeing, and that the fish will be happy, but not wishing to buy a load of expensive plants and have half of them melt, I thought that I’d give the dark start method a try. I filled the tank, making sure to use a dechlorinator, and have heated the water to 24 degrees.

The tank has been set up like this with the light off for 8 days now and I have been testing the water every day with an API drop test kit.

The results on the first day were as I expected. Ammonia high, at 4 ppm (presumably leaching out of the soil), nitrite at 0, and nitrate at 80 (seemed a little high). On the second day the nitrite rose to 0.25, which made me think that the nitrification bacteria had already started, but then nothing changed. The nitrite has stayed at 0.25 ppm since day 2, and the ammonia has not dropped. The API ammonia test is not very clear after 4 ppm but on a couple of days the level seemed perhaps to look more like 8 ppm so I have done two 15% water changes so that the ammonia levels do not inhibit the bacteria, but it has remained at 4 ppm.

I am probably being impatient, but I had expected to see some increase in nitrites by day 8. Is this normal?

Another mystery relates to my PH and water hardness. I live in London where the tap water is extremely hard; out of the tap I get a PH of 7.5, KH of 215 and GH of 340. What I don’t understand is why my tank water results are very different. I have tested PH every day and it has been 6.6 seven days out of eight, and 6.4 on one day. Further to this, the GH of the tank water is 161; I have tested this twice and it has remained steady. I’m not sure that this relevant to the nitrification bacteria in my tank, but I’d be interested to hear an explanation if anyone has one. The wood might explain the lower PH, but how does hardness reduce? When I tested the tap water I had assumed that I’d either need to add reverse osmosis water in my water changes, or only buy fish which tolerate very hard water, but if the lower hardness conditions in my tank remain after planting, do you think it would be safe to purchase less tolerant fish?

As far as I can tell, all of the conditions in my tank are suitable for nitrification of the ammonia to begin, so why isn’t it?

Happy to post the full test results if it'd be helpful

Thanks for your help and for providing such an amazing resource for novices such as myself.

Nick

 

[_Maq_]

The substrate you've opted for contains PROTONATED CLAY. Thanks to cation exchange capacity it has begun binding Ca2+ and Mg2+ cations and releasing H+ cations (protons). That's why your pH and hardness are decreasing substantially. In a few months, this ability of the substrate will get exhausted.
Nitrites should peak approximately on 20 to 30th day.
I highly appreciate your 'beginner's' approach. If only all beginners were that thoughtful!

 

[Ria95]

It probably has to do with the substrate ‘buffering‘/lowering the GH and KH of the water. And is also possibly why you see the delay in nitrification. I would suggest getting a KH test kit as well. Alternatively, just get some baking soda (NaHCO3) and dissolve 1 teaspoon in some water and add to the tank. See if it picks back up. If it‘s 0 , you might need to dose regularly until the tank deals with the ammonia. In the long term the buffering capacity will be lowered or stop.

 

[nick elton]

Hi, thanks for your input

The substrate you've opted for contains PROTONATED CLAY. Thanks to cation exchange capacity it has begun binding Ca2+ and Mg2+ cations and releasing H+ cations (protons). That's why your pH and hardness are decreasing substantially. In a few months, this ability of the substrate will get exhausted.
Nitrites should peak approximately on 20 to 30th day.
I highly appreciate your 'beginner's' approach. If only all beginners were that thoughtful!

Thanks for explaining the hardness decrease. Sounds like unless I want to be dealing reverse osmosis filtering in the future, I'd better stick with hard water tolerant fish, as the hardness will creep back up.
I guess I just have to be patient with the nitrification. I added Fluval Cycle in the first couple of days, but I suppose this is further evidence that bacteria in a bottle doesn't really work to speed things up.

It probably has to do with the substrate ‘buffering‘/lowering the GH and KH of the water. And is also possibly why you see the delay in nitrification. I would suggest getting a KH test kit as well. Alternatively, just get some baking soda (NaHCO3) and dissolve 1 teaspoon in some water and add to the tank. See if it picks back up. If it‘s 0 , you might need to dose regularly until the tank deals with the ammonia. In the long term the buffering capacity will be lowered or stop.

Thanks for your input. I have a KH test and had previously tested the tap water at 215 ppm. I just tested the KH of the tank water and was pretty surprised to see it was at 36 ppm (2 dKH). Is this low of a KH likely to stall the bacteria? I can try the baking soda and see if the increase in KH picks things up.

 

[dw1305]

Hi all,
Welcome to UKAPS,

I am probably being impatient, but I had expected to see some increase in nitrites by day 8. Is this normal?

Have a look at <"Seasoned Tank Time">, it should answer some of your questions. I understand this is going to be difficult with your son, but it gives you the best chance of long term success and hopefully saves the <"heart-break of dead fish">.

Personally I'd probably ignore the results of the test kits for a couple of weeks, despite what you will read etc <"they aren't very accurate">.

I can try the baking soda and see if the increase in KH picks things up

Carry on changing some water (at least 50% a week), it will supply plenty of bicarbonate ions (from the dissolved chalk) and eventually the tank will stabilise and you can plant it. I'm not a baking soda (NaHCO3) fan because of the sodium (Na). If you feel that you need to raise hardness (and I'm pretty sure you don't) then potassium bicarbonate (KHCO3) would be better, you can get it as "food grade" easily via Amazon etc.

I'm going to suggest at least a month for the dark start and then another month <"to allow the plants to grow in">. You can always add some <"tank janitors"> to the tank once it is planted. PM me nearer the time and I'll sort some out (and <"some low. tech plants">) for you and your son.

<"Cycling isn't really a linear process">. You won't necessarily get a rise in nitrite (NO2-) levels, because we now know that COMAMMOX Nitrospira (that oxidise ammonia (NH3) directly to nitrate NO3-) are very important in aquarium nitrification.

cheers Darrel

 

[nick elton]

Personally I'd probably ignore the results of the test kits for a couple of weeks, despite what you will read etc <"they aren't very accurate">.

Carry on changing some water (at least 50% a week), it will supply plenty of bicarbonate ions (from the dissolved chalk) and eventually the tank will stabilise and you can plant it. I'm not a baking soda (NaHCO3) fan because of the sodium (Na). If you feel that you need to raise hardness (and I'm pretty sure you don't) then potassium bicarbonate (KHCO3) would be better, you can get it as "food grade" easily via Amazon etc.

I'm going to suggest at least a month for the dark start and then another month <"to allow the plants to grow in">. You can always add some <"tank janitors"> to the tank once it is planted. PM me nearer the time and I'll sort some out (and <"some low. tech plants">) for you and your son.

<"Cycling isn't really a linear process">. You won't necessarily get a rise in nitrite (NO2-) levels, because we now know that COMAMMOX Nitrospira (that oxidise ammonia (NH3) directly to nitrate NO3-) are very important in aquarium nitrification.

cheers Darrel

Thanks Darrel, that's very helpful. I'll happily add potassium bicarbonate if I need to, but as the KH of my tap water is so high, I'm sure you'r right that if it genuinely is the low KH that's slowing down the cycling, doing a big water change should bring it back up enough to get things moving.

Thanks for your offer regarding tank janitors and plants. I will be sure to be in touch once the tank seems settled.

Interesting to hear that the test kits aren't very accurate. I guess the only critical one is the ammonia, as the levels my tests are showing clearly indicate that the tank is not safe for fish yet.

 

[dw1305]

Hi all,

Interesting to hear that the test kits aren't very accurate.

It depends a little bit on <"what you are testing for">, some <"kits are quite accurate"> in nearly all circumstances, some aren't.

I guess the only critical one is the ammonia,

Ammonia (NH3) and nitrite (NO2-) are the two "biggies", just because they are compounds <"that are toxic to fish"> at very low levels. Nitrite testing is relatively straightforward, but there are <"some issues"> with ammonia / ammonium (<"Total Ammoniacal Nitrogen (TAN)">) testing.

as the levels my tests are showing clearly indicate that the tank is not safe for fish yet.

I want <"belt and braces">, <"plenty of plants, plenty of oxygen"> and a <"diverse microbial assemblage"> to try and keep ammonia levels low at all times.
I don't really care how people get there, but plant / microbe biofiltration (<"phytoremediation">) is definitely my weapon of choice.

I don't test for them in the aquarium, we actually <"have the kit"> for most water testing methods, but <"I'm pretty confident"> (or absolutely delusional) that I have techniques that mean that high levels of ammonia are never going to be a problem for me.

In most freshwater <"we use a biotic index as our weapon of choice">, and if that showed an unexpected reduction in water quality we would run samples through the analytical equipment.

cheers Darrel

 

[Ria95]

If your tap water is already so rich in KH then just do the water change. The soil will provide plenty of ammonia again to keep things going. A KH of 2 should still allow them to grow but most report better performance at higher values. Most active nitrifying bacteria will use the carbon from the HCO3+ neutralizing the aciditty created.

 

[_Maq_]

Don't fear of low bicarbonates. I have routinely <0.3 °dKH in my tanks and nitrification is running very well, even in acidic territory (pH = 5 to 6).
However, what DOES hinder nitrification is the competition of heterotrophic microbes. Decomposers. So, the fastest cycling can be achieved without any organic stuff, just mineral salts, incl. ammonia, of course.

because we now know that COMAMMOX Nitrospira (that oxidise ammonia (NH3) directly to nitrate NO3-) are very important in aquarium nitrification.

Are they?
For decades, crowds of hobbyists worldwide detected nitrite spike. And now - you say they were wrong?
Me, I have made many diligent experiments about cycling (using good analytical methods), and never ever detected anything like comammox. (While I did detect the delay when adding organic matter and/or "beneficial bacteria", which perplexed me a lot. Only later I've found a paper which explained it.)

 

[dw1305]

Hi all,

Are they?
For decades, crowds of hobbyists worldwide detected nitrite spike.

It isn't something I've ever looked at but it is the conclusion from:

• <"Microbial community analysis of biofilters reveals a dominance of either comammox Nitrospira or archaea as ammonia oxidizers in freshwater aquaria">.
• <"Ammonia-oxidizing archaea and complete ammonia-oxidizing Nitrospira in water treatment systems">.
• <"https://journals.asm.org/doi/full/10.1128/mSphere.00143-19">. "Component Microenvironments and System Biogeography Structure Microorganism Distributions in Recirculating Aquaculture and Aquaponic Systems"
• <"Taxonomic and functional profiling of nitrifying biofilms in freshwater, brackish and marine RAS biofilters">

cheers Darrel

 

[MichaelJ]

I really have nothing to say on the matter at this point (I am a slower reader), but just to remind us all: Comammox stands for Complete Ammonia Oxidiation... It essentially refers to the micro organisms that ultimately converts ammonia into nitrate - what we call nitrification - its the good guys that lives in our substrate and filters - although in a planted tank the amount that lives in the filter is insignificant compared to the substrate (different story).

The Nitrospira and their role, that Darrel is referring to was not discovered and understood until quite recently...and the understanding is still evolving I suppose.

Cheers,
Michael

 

[_Maq_]

It isn't something I've ever looked at but it is the conclusion from:

Thanks, Darrel. Interesting reading.

and the understanding is still evolving I suppose.

Precisely. That's why I'm quite cautious to take any conclusions. It seems that nitrifying microbes are a much richer group than previously thought, both generally and in our tanks specifically. Also, I believe that even non-significant differences in environment may have significant impact on the composition of nitrifying community. Thirdly, when it comes to comammox bacteria, I fully admit they may play a significant role in our tanks, however it seems that they do not tend to develop very quickly. That's why we observe nitrite spike in the beginning, while cycling.
Beside that, I fully agree that the tank itself - in contrast to filters - is the decisive habitat for our microbes, incl. nitrifiers. I've got rid of my filters completely a while ago, and have got plenty of experience with running various tanks without filters. Many environmental parameters vary widely (most importantly pH), yet both decomposition and nitrification may be surprisingly effective.
In sum, I tend to restrict myself to the term "nitrifying microbes" because I've got no way to tell whether they are bacteria or archaea, which ones of them, performing comammox or two-step nitrification, etc. We simply don't know. We can only tell whether nitrification runs or not.
Adding to that, there's quite a mystery around denitrification and DNRA (dissimilatory nitrate reduction to ammonia) in our tanks. We have good reasons to believe that they commonly happen, yet evidence is missing (as far as I know). I'd really love to arrange a test which would gather evidence of denitrification, but I'm short of ideas how to do it. If you have a suggestion, please let me know.

 

[dw1305]

Hi all,

It seems that nitrifying microbes are a much richer group than previously thought, both generally and in our tanks specifically.

Yes I'd guess that <"we are still right at the start"> of discovery of <"novel nitrifying organisms">.

Once people had the <"gene sequences that code for Ammonia Oxidation"> then it became apparent that they were a lot more common and widespread than the traditional view allowed. I'll be honest <"I wasn't at all surprised">, and would actually have been a lot more surprised if they hadn't found them.

Also, I believe that even non-significant differences in environment may have significant impact on the composition of nitrifying community.

I'm sure <"you are right">. This thread has a lot more scientific references in it, <"Bacteria/biological starters">, it is quite a long thread, but worth reading.

.......... All the work I've read would strongly suggest that even small increases in ammonia loading drastically effect the microbial community. The initial reference would be Bartelme et al. (2017) <"Freshwater Recirculating Aquaculture System Operations Drive Biofilter Bacterial Community Shifts around a Stable Nitrifying Consortium of Ammonia-Oxidizing Archaea and Comammox Nitrospira"> Front Microbiol. 2017; 8: 101. This had a top level of about 14 µM NH3......

Thirdly, when it comes to comammox bacteria, I fully admit they may play a significant role in our tanks, however it seems that they do not tend to develop very quickly. That's why we observe nitrite spike in the beginning, while cycling.

This may well be one of the reasons why an extended growing in period (<"Seasoned Tank Time">) works, and also why adding established filter media (and probably <"plant / substrate as well">) shortens that time period.

This was what Dr Newton said: <"Correspondence with Dr Ryan Newton - School of Freshwater Sciences, University of Wisconsin—Milwaukee">.

......... If you do need to add nitrifiers the best source is from an aquaponics or aquaculture system that is already running and removing ammonia. Some water or sediment/soil or part of the biobilter (if there is one) is an excellent starter. Without this source as an inoculum then you could add some roots from plants from any other tank that is running - these are likely to have nitrifiers associated with them. A small clipping put into the tank would be enough.

In some lab tests we found that adding previous material from a running biofilter could reduce ammonia oxidation start-up time from 2-3 weeks to 2-3 days. We also tested a commercial product of nitrifiers & it did decrease the time to ammonia oxidation start-up. It was slower than our biofilter material transfer, but much quicker than doing nothing. However, the microbes present in the system from the commercial product disappeared over a few weeks and were replaced by those more common to our system. So, it seems some products could help “jump-start” the process, but it will be a lot less predictable and ultimately may not determine what microbe succeed in the long run.......

Which led onto:

It would be interesting to know how long these microbes can survive outside of their intended environment.

That is an interesting question. It looks like COMAMMOX Nitrospira and Ammonia Oxidising Archaea (AOA) are pretty much universal in low ammonia situations, but that could be because,

• they are very efficient at finding these resources, or
• it could be because they've had a very long time (most of the last 3.5 billion years) to become universal or
• a combination of both factors

And I don't know the answer to that one, again I'd guess it is a mixture of both factors, but I have no idea of the relative proportions of "found it" to "sat waiting".

In sum, I tend to restrict myself to the term "nitrifying microbes" because I've got no way to tell whether they are bacteria or archaea, which ones of them, performing comammox or two-step nitrification, etc. We simply don't know.

I think "nitrifying microbes" is fine as a term, as you say we've got no way of knowing what they are.

there's quite a mystery around denitrification and DNRA (dissimilatory nitrate reduction to ammonia) in our tanks. We have good reasons to believe that they commonly happen, yet evidence is missing (as far as I know). I'd really love to arrange a test which would gather evidence of denitrification, but I'm short of ideas how to do it. If you have a suggestion, please let me know.

Not easily for us, I think it would have to be via <"stable nitrogen isotopes">, ideally supplied in the initial ammonia (NH3) addition.

This is a plant one from phytoremediation <"Use of stable nitrogen isotopes to track plant uptake of nitrogen in a nature-based treatment system">*, but there are plenty of wastewater ones that look at the complete nitrification / denitrification arc.
<"Using stable isotopes to identify nitrogen transformations and estimate denitrification in a semi-constructed wetland">**

* Cecchetti AR, Sytsma A, Stiegler AN, Dawson TE, Sedlak DL. (2020) "Use of stable nitrogen isotopes to track plant uptake of nitrogen in a nature-based treatment system". Water Res X. 16:9:100070
** Yuming Hu, Zanfang Jin, Qiyue Hu, Jing Hu, Chenhao Ni, Feili Li, (2020) "Using stable isotopes to identify nitrogen transformations and estimate denitrification in a semi-constructed wetland" Science of The Total Environment, 720.

cheers Darrel

 

[nick elton]

Hi all,

Thanks for your advice. There were differing opinions about how important KH is in bacteria colonisation but I figured that water changes would only help and so since my last post 10 days ago I've done three 50% water changes, roughly whenever the dKH reading drops to 3.

Whatever the cause, there is a definite trend towards reducing ammonia and nitrites, but the pattern is nothing like I would have expected. Is anyone more experienced than me able to look at the graphs below and take a guess as to what is going on, and whether my tank is nearly cycled?

The API test kit is not the easiest to read, and there were certainly some occasions where I couldn't be 100% sure of the reading. I also understand that there is an inherent inaccuracy referred to previously.

Nevertheless the general trend definitely seems to be reducing ammonia, and in the last two days the nitrites have completely dropped off and are now immeasurable. Both of these seem like good things, but the pattern has not been as I expected, and I am surprised to see that nitrites have dropped off before ammonia.

Is this anything to worry about, or would you expect ammonia to follow suit shortly?

Looking forward to getting some plants in the tanks so that I can stop staring at a dark tank and head towards finally getting my sone a populated fish tank!

Thanks again,

Nick

 

[dw1305]

Hi all,

There were differing opinions about how important KH is in bacteria colonisation but I figured that water changes would only help and so since my last post 10 days ago I've done three 50% water changes, roughly whenever the dKH reading drops to 3.

Personally I'm not too worried about <"the dKH level">, but I'm always <"keen on water changes">.

Whatever the cause, there is a definite trend towards reducing ammonia and nitrites, but the pattern is nothing like I would have expected. Is anyone more experienced than me able to look at the graphs below and take a guess as to what is going on, and whether my tank is nearly cycled? ........ The API test kit is not the easiest to read, and there were certainly some occasions where I couldn't be 100% sure of the reading. I also understand that there is an inherent inaccuracy referred to previously.

I'll be honest I'd take the readings with a pinch of salt, but in some ways it doesn't matter if you are "cycled" or not. I'd probably add a floating plant now, they aren't CO2 limited and can make use of elevated nitrogen levels. They also won't get algae on their leaves, although they may do on their roots.

"Nile Cabbage" Pistia stratiotes would be a good one, they use it a lot in <"tropical wastewater treatment">, but <"Duckweed"> (Lemna minor) from a local pond would work just as well. If you want Pistia (or other floating plants) I have spare plants.

cheers Darrel

 

[nick elton]

I'll be honest I'd take the readings with a pinch of salt, but in some ways it doesn't matter if you are "cycled" or not. I'd probably add a floating plant now, they aren't CO2 limited and can make use of elevated nitrogen levels. They also won't get algae on their leaves, although they may do on their roots.

"Nile Cabbage" Pistia stratiotes would be a good one, they use it a lot in <"tropical wastewater treatment">, but <"Duckweed"> (Lemna minor) from a local pond would work just as well. If you want Pistia (or other floating plants) I have spare plants.

cheers Darrel

Thanks Darrel, I like the idea of floating plants, but I'm a bit worried about Pistia stratiotes given the small size of my tank. I like the look of Phyllanthus fluitans, I'm aware it won't be red in my tank given the high nitrates at the moment, but do you think that it would do a similar job of helping the tank settle in before I add other plants?

 

[dw1305]

Hi all,

Thanks Darrel, I like the idea of floating plants, but I'm a bit worried about Pistia stratiotes given the small size of my tank.

I wouldn't worry, if it does well it will produce loads of offsets and you can just remove the mother plants all time. It doesn't grow enormous like in the Amazon in our tanks.

I like the look of Phyllanthus fluitans, I'm aware it won't be red in my tank given the high nitrates at the moment, but do you think that it would do a similar job of helping the tank settle in before I add other plants?

Yes should do, floating just means it has access to aerial CO2.

It isn't a plant that I've had any joy with.

Cheers Darrel

 

[nick elton]

Hi all,

My water parameters were all heading in the right direction, so on day 25 I planted the tank up. There's a mixture of slow and fast growing plants and they generally seem to be doing well. The Blyxa Japonica was a tropica 1-2-grow from Horizon Aquatics which was in poor shape when it arrived, this combined with a planting position which probably doesn't have enough light, and it's showing a fair amount of melt. Everything else is doing ok, and the stem plants particularly are shooting up (ludwigia super red, rotalia rotundifolia and limnophila sessilliflora).

I went for Salvinia for the floating plants. I was concerned that the humidity and drips under the lid would trouble it, but it is growing at an absurd rate. If it carries on growing like this, I'll be removing a load of it weekly as it's already casting a lot of shade. The only issue I've had with the floaters was in relation to the filter. The Fluval U2 has three output modes, and I had previously set it on the vertical spray bar, which made strong but gentle flow around the tank. This mode created too much surface movement and the Salvinia was getting blasted around. The filter now comes out of the bottom output which creates less even flow around the tank, and really hammers the stem plants around at the back.

Over the last few days the ammonia has started heading toward zero, and today (day 29) I had my first zero reading. I was planning to pick up some snails and shrimp this weekend, but my Nitrite levels have come back off of zero and are now showing 0.25 ppm again. I'll give it a few days and see what happens. Any one want to hazard a guess why this might have picked up again? Additional organic load from melting plants overloading the still establishing bacteria culture?

Perhaps I'm misinterpreting it, but the buffering effect of the aqua soil seems to be more short lived that I expected. Even accounting for the drops in kH after water changes, there is a clear trend for rising kH in the tank (the rising PH trend is even more clear cut). At this rate, it won't be long before the tank hardness is the same as the tap water.

Water chemistry readings for anyone interested

I can see already that cleaning the glass around the internal filter and heater is going to be a bit of a pain, and I'm considering getting a small external canister filter to replace it. Do you think if I ran the two filters side by side for a few weeks, the new filter would cycle effectively and allow me to remove the old one. Is there any sensible way to know when it would be a good time to remove the old filter?

Thanks again!

Nick

 

[X3NiTH]

I'm considering getting a small external canister filter to replace it. Do you think if I ran the two filters side by side for a few weeks, the new filter would cycle effectively and allow me to remove the old one.

Yes you can do that or alternatively take the cycled media from the internal filter and put it in the canister filter and you are good to go.

 

[dw1305]

Hi all,

I went for Salvinia for the floating plants. I was concerned that the humidity and drips under the lid would trouble it, but it is growing at an absurd rate.

Perfect. It is just telling you there are a lot of nutrients <"available to it">. <"https://www.researchgate.net/public...c_Fern_with_Potential_Use_in_Phytoremediation">*

I was planning to pick up some snails and shrimp this weekend, but my Nitrite levels have come back off of zero and are now showing 0.25 ppm again. I'll give it a few days and see what happens. Any one want to hazard a guess why this might have picked up again?

I'm going to guess that it isn't an actual NO2- reading, plants are very effective at removing all kinds of <"fixed nitrogen">, including nitrite (NO2-).

Additional organic load from melting plants overloading the still establishing bacteria culture?

I'd guess not, the small amount of melting Blyxa isn't going to supply much protein, and it is that protein that <"would be microbially dismembered">, and the resulting ammonia (NH3 / NH4+ (TAN)) oxidised to NO2- and eventually nitrate (NO3-), if the plants don't grab it first.

The only issue I've had with the floaters was in relation to the filter. The Fluval U2 has three output modes, and I had previously set it on the vertical spray bar, which made strong but gentle flow around the tank. This mode created too much surface movement and the Salvinia was getting blasted around. The filter now comes out of the bottom output which creates less even flow around the tank, and really hammers the stem plants around at the back.

That might possibly be relevant, if dissolved oxygen levels were now limiting nitrification, but I'm 99.9% sure that isn't the case, <"purely because of the plants">.

I was planning to pick up some snails and shrimp this weekend

I'm sure you will be fine. If you were worried you could start with some <"low cost hardy snails">.

Perhaps I'm misinterpreting it, but the buffering effect of the aqua soil seems to be more short lived that I expected. Even accounting for the drops in kH after water changes, there is a clear trend for rising kH in the tank (the rising PH trend is even more clear cut). At this rate, it won't be long before the tank hardness is the same as the tap water.

I think you are right, it is <"ion exchange"> so it won't take long with hard tap water for all the H+ ions on the substrate to be exchanged for calcium (Ca++) ions from the tank water.

* Dhir, Bhupinder. (2009). "Salvinia: an Aquatic Fern with Potential Use in Phytoremediation". Environ We Int J Sci Tech. 4.

cheers Darrel