Strange water parameters after switching from rainwater to RO

[Vsevolod Stakhov]

Hello, I have some weird issues with my water in my softwater planted tank with Colombo soil. I always add around 75-80% of the rainwater and mix it with tap water (hard 110ppm Ca, 12dKH). TDS of my rain water was around 5-7ppm and TDS of the RO water is around 15ppm. TDS in the tank is around 168ppm.

After changing the basis water from rain to RO, I observed that all my crystal shrimps have died. Cherry shrimps are also not very happy from what I see. I have measured water parameters and they are quite confusing. PH with CO2 is 6.8 but when I remove extra CO2 by shaking it raises up to 7.6, with dKH=2-3. In another planted tank that has exactly the same dKH I have PH with CO2 equal to 6 and PH without CO2 equal to 6.8 (which is totally reasonable). I'm trying to understand why do I have such a high PH now, as with rain water it was not higher than 6.

One hypothesis is that the lower PH in the large planted tank is caused by phosphates buffer, as that tank has ~2ppm of PO4 according to the API test kit. Smaller tank has 0.25ppm of PO4 (that could be explained by aquasoil).

I see no visible issues with fish and plants (R. macrandra, fissidens, monte carlo etc), but I cannot understand why I have that PH rise and what has happened with my shrimps. Any ideas are much appreciated - I can provide more information if needed.

 

[jaypeecee]

Hello, I have some weird issues with my water in my softwater planted tank with Colombo soil.

Hi @Vsevolod Stakhov

I just had a very quick look around before I need to water the garden!

Is the following of any relevance to what you are reporting:

Colombo Florabase substrate review

www.practicalfishkeeping.co.uk

I also noted the following:

"This product looks good, feeds plants and can lower pH".

"With the potential to release ammonia this isn’t a product for beginners..."

JPC

 

[Vsevolod Stakhov]

"This product looks good, feeds plants and can lower pH".

Yes, but in my case PH is much higher than PH in the nearby tank with the neutral soil and the same RO system and the same dKH. I really don't understand why it can be the case (well, aside of PO4 concentration...).

"With the potential to release ammonia this isn’t a product for beginners..."

There are two soils: one with nutritients and one without. I have one that has no nutritients. Anyway, this tank is one year old, it was properly cycled on ammonia prior to anything, it has decent filtration and I actually use fertilizers with ammonia. So I'm not scared of ammonia, but I'm, well, scared about such an unpredicted PH rise for no apparent reason (dKH is still quite low).

 

[Hufsa]

I may be completely missing the plot here but is it possible that the switch to uber clean RO has caused something to leach out of the substrate?

 

[Vsevolod Stakhov]

I may be completely missing the plot here but is it possible that the switch to uber clean RO has caused something to leach out of the substrate?

The TDS at the output of the RO unit is slightly higher than for rainwater butt: like 15ppm vs 7ppm. Anyway, I have slightly a different hypothesis now: when rainfall got depleted I've added some RO water directly to the water butt disturbing the long lived mud on the walls and at the bottom. Maybe there were some toxic chemicals, such as pesticides or other biocides buried in that mud? That could explain sensitive shrimps deaths but it still does not explain the higher PH comparing to the neutral substrate tank.

 

[tam]

RO water should really be <10 - ideally pretty close to 0. It might be your filters need changing. The rain could have some hardness in it (rain isn't necessarily zero everything), so rain isn't necessarily the same as RO. How accurate is your mixing 20% tap would give you 2.4gh - which I think is a little low for shrimp? If your rain had some gh maybe that was enough to just bring it up enough for them. If you then added RO to it, you'd have reduced the gh in the rain and ended up with a lower gh.

What's your gh in the tank? The TDS reading might seem fine, but it may be made up of all sorts of 'stuff' that isn't gh. In fact your tank TDS seems very high if you are using 80% RO/rain.

 

[Vsevolod Stakhov]

RO water should really be <10 - ideally pretty close to 0.

TDS meters actually measure conductivity, not the real TDS. Hence, even dissocated CO2 (as H+ and HCO3- ions) influence on those readings. So I'm not really blaming my RO membrane.

How accurate is your mixing 20% tap would give you 2.4gh - which I think is a little low for shrimp?

My tap water has 12dKH and 18-19dGH + I also add some MgSO4 (to 5ppm Mg) during water preparation. So dGH is around 5, which is totally fine for shrimps I suppose.

In fact your tank TDS seems very high if you are using 80% RO/rain.

It's probably because of EI like fertilization mode and CO2 injection. It has always been around this value.

One thing I really don't understand is the PH difference with a tank that has neutral substrate (that also uses the same RO water + tap water mix). Soil tank PH must be lower and it was lower in the past. So why could it change, counting that KH is the same?

 

[Vsevolod Stakhov]

Plants and snails also look fine, that's just shrimps that were affected:

 

[_Maq_]

TDS meters actually measure conductivity

Yes, and I find it quite surprising that TDS measurements appear in this forum quite often. TDS meters do measure conductivity and transform the number to TDS by using some factor chosen quite liberally. In the end, it says pretty nothing.
(I'm intrigued by your problem but have no ideas, suggestions. It's really weird. Could it be that, p.e. after long period of inactivity, your RO machine got somehow "bad"?

 

[Vsevolod Stakhov]

Ok, I have started a simple expreiment: I've added soil to a small container, washed it with RO water and filled the container again with RO water. I have used another container as a reference:

Initially my reference sample has TDS=28, so it seems that my RO membrane needs to be replaced, but it also depends on the source water temperature (it is hot now) and the source pressure (the pressure is low). The sample with soil behaves differently. Initially TDS was equal to 33, in one hour it was 42 and after another hour it was 55. So something definitely leaches from the soil. I will wait for some more time unless tds stops changing and try to check kh/gh/ph...

 

[Vsevolod Stakhov]

I will wait for some more time unless tds stops changing

95ppm today That's really worrying.

 

[_Maq_]

That's really worrying.

Yet not that surprising, at least to me.
Still, if I'm not mistaken, the soil remains the same. It's the water which turned from rain to RO. I'd suppose both should react with the soil in similar fashion. Perhaps exchanging protons for some metal cations at cation exchange sites of the clay which forms a part of the soil. I'm still short of ideas what made such a striking difference.
Have you checked for all possible flaws, like invalid measurements, defect RO machine, etc.?

 

[dw1305]

Hi all,

Yes, and I find it quite surprising that TDS measurements appear in this forum quite often. TDS meters do measure conductivity and transform the number to TDS by using some factor chosen quite liberally. In the end, it says pretty nothing.

This has come up a <"few times on the forum"> and <"conductivity in microS"> is a much more sensible unit.

I think the problem is a bit like using "dGH" and "dKH". The aquarium industry uses them, and "ppm TDS" etc., even though they <"are bizarre derived units"> and microS and mg / L would make a lot more sense.

cheers Darrel

 

[John q]

95ppm today That's really worrying.

Have you tried the same experiment with rain water?

 

[tam]

Is that existing substrate taken out of your tank or new out of a bag? I'd expect it to produce less after its been in a tank awhile.

Also, are you dechlorinating still - you don't know what's coming through your RO if you are getting high tds?

 

[Vsevolod Stakhov]

Perhaps exchanging protons for some metal cations at cation exchange sites of the clay which forms a part of the soil.

The RO water has not many free H+ cations (1*10-7 in molar concentrations). Dissolved CO2 gives some but not many. But if soil exchanges protons with, let's say, K+, then PH raise is clearly here. But I was under impression that the soil should do the opposite exchange or it is kinda... useless.

It's the water which turned from rain to RO.

It might be a coincidence and unrelated thing. Shrimps could die because of something washed out from the rainwater butt and PH could just gradually increase over time (I've not measured it constantly tbh).

Have you tried the same experiment with rain water?

I have some small amounts of rainwater left. But it is duluted with RO water and it has TDS=25 due to heavy evaporation

s that existing substrate taken out of your tank or new out of a bag

It is the the existing substrate washed in RO water initially to remove dirt and mug.

Also, are you dechlorinating still

It seems to be useless for RO as there is a carbon filter before RO membrane. Also, free chlorine in form of ClO ions affects conductivity, so it is not possible to have any decent amount of those ions with that TDS readings. Chloramine, on the contrary, has likely no chances to pass RO membrane due to it's molecule size.

 

[John q]

is duluted with RO water and it has TDS=25 due to heavy evaporation

I suspect that the ro unit is somehow adding kh? surely that's the only explanation? I think the substrate is a red herring on this occasion. Hence the need for testing with rain water.

 

[John q]

Also, free chlorine in form of ClO ions affects conductivity, so it is not possible to have any decent amount of those ions with that TDS readings.

1ppm of free chlorine will kill your fish... tds goes from 152 to 153.... is that leaf melt, saliva thats dribbling from your chin or chlorine...

 

[Vsevolod Stakhov]

I suspect that the ro unit is somehow adding kh?

Hold on, RO unit apparently skips some HCO3 ions and it has non zero KH. But you cannot find a single dKH in RO water using aquarium tests (that are very precise btw) - I've tested it many times. So my quality check of an RO unit is simple: if KH probe is red from the first drop, then RO unit works (of course, it is true merely for my hard tap water). So the final dKH of RO unit is <1. Otherwise, it is a clear sign that you need to replace membrane.

1ppm of free chlorine will kill your fish

I have no doubts that 1ppm of free chlorine will kill fish and shrimps. I've even tested that on my daphnia cultures (those cultures could survive 60ppm of total NH3+NH4 at PH=8, but 1ppm of free chlorine has killed all of them in 48 hours). However, my tap water has around 0.1ppm of free chlorine and RO unit will filter at least 90% according to PPM and the ionic radius of the HClO (it is not B!). So I'd exclude free chlorine here...

The only parameter that I cannot understand is PH: why PH on a soil based tank is higher than on a neutral substrate with the same dKH. And this PH difference cannot be explained by CO2 levels difference, as I have shaked samples for quite a long to ensure that CO2 is in equilibrium with the atmosphere.

 

[dino21]

A basic RO system will not produce 0 tds in a hard water area as its only about 95-99% effective.
In such cases you need a add, if not already fitted, a fourth pod using a DI resin to remove those last traces.
If you do have DI fitted it may be just a case of renewing or 'cleaning' the resin,
How old is the RO membrane and how much water are you running though it a week ?
Do you have and use a flush kit on it, before each session ?
Chlorine should be removed by the Carbon filter as it can damage the RO membrane, again how often do you change the carbon and the pre filters ?