amount of C02 required based on KH / mixing RO with tap water

[Paul195]

Hi

I asked the guy at the LFS about their planted tank, which is using RO water, and I noticed that they were injecting C02 at a very slow rate; something like 1 bubble every two seconds, whereas when I was using C02 previously, I was having to pump it in at something like 6-8 bubbles per second to achieve a 1 PH drop to get C02 levels right. My tank parameters are PH 8.4, KH 11, GH 13.

Could somebody explain the relationship between the the PH, KH and the amount of C02 to reach the desired C02 levels please ?

Also, If I were to mix my current tap water with the above parameters,50/50 with RO, what would my resulting parameters be ? am I right in saying that my KH & GH would half ? what about PH ?

Thanks
Paul

 

[Zeus.]

Hi Paul
The relationship is simple and complex, depends on your understanding of chemistry

RO water has nearly everything stripped from it so it is more or less just water. (Distilled water is 100% pure) So when you add CO2 to it the [CO2] increases very fast so you dont need to add much to get the desired [CO2]. The reason is RO distill water has no (or very few buffers)
Tap water contains salts/minerals which act as buffers, so when you inject CO2 not all of it is available as CO2 for the plants it is buffered by the dissolved minerals.. So you have to inject more CO2 to get the same [CO2] as RO water.

Yes if you mix your tap water and RO as you suggest it would be about half, Your pH would be less.

But if you use 100% RO water controlling the [CO2] can be very tricky as you can go from ideal levels to toxic to livestock very quick. Where as Hard water (high Kh Gh) you will use LOTS more CO2 getting to your target pH.

I live in a hard water area and go though 6.5Kg of CO2 a month on my 500l tank. But I do have it at very high [CO2].

Very hard to compare one tank to another esp when talking BPS, mine is insanely high but it works well for me.

Hope that is of some help.

Zeus

I was going to put the graphs etc but I'm sure you have seen them, sometimes less is more

 

[naz]

I have extremly hard water living in the south east england,my kh is about 27 dkh,iv been in to the planted tank hobby for the last 15 years, always had a planted tank going in one way or another.Just use staight 100% ro water with a gh booster to keep the gh betweem 3 and 5 so you have some mag and calcium this is how i run all my tanks......and reap the rewards...

 

[Paul195]

Thanks for both of your replies. Funnily enough Naz, this is the decision that have have just made after visiting the local fish store.

Could you recommend any specific product to remineralize with or are they all ok to get the job done ? I've heard a lot about equilibrium.

I am planning to take down my current tank and start from fresh with new substrate, plants and RO. To do this I will obviously have to house my current stock (1 SAE, 2 oddessa barbs, 2 dwarf neons, 4 otto cats) temporarily, and I'm a little confused about how to transition the the stock to the new tank. The guy in the LFS said that generally fish will fairly easily adapt to softer water, he said they have a hard time when put in harder water. With this said, I assume that I can't just take them out of my tap water and put them straight into remineralized RO water? Could I do this if I were to drip acclimatize them over the space of a few hours or would this need to be done over a longer period of time / using a different method ?

I have read that you can do it by just doing small partial water changes replacing with RO, but I'm a bit unsure when starting a new tank with 100% RO.

 

[naz]

hi paul
i use seachem equilibrium in all my tanks to raise the gh,i have a ro unit and i use 27l plastic containers that i fill up then i dose 3/4 tsp of equilibrium to the container then just take 27l of take water out and replace,this keeps my gh at about 3 degrees then i dose fert once a week a water change.

i would just drip acclimatize them over a period of 2 hours or so and thay will be fine,this is how i do it.

 

[Zeus.]

Also the size of the tank may be a limiting factor, I have over 250l to change a week, thats a lot to store and treat. RO wise it would cost a fortune in water. So I just accept the hard water and deal with it

 

[ceg4048]

Hi

I asked the guy at the LFS about their planted tank, which is using RO water, and I noticed that they were injecting C02 at a very slow rate; something like 1 bubble every two seconds, whereas when I was using C02 previously, I was having to pump it in at something like 6-8 bubbles per second to achieve a 1 PH drop to get C02 levels right. My tank parameters are PH 8.4, KH 11, GH 13.

Could somebody explain the relationship between the the PH, KH and the amount of C02 to reach the desired C02 levels please ?

Also, If I were to mix my current tap water with the above parameters,50/50 with RO, what would my resulting parameters be ? am I right in saying that my KH & GH would half ? what about PH ?

Thanks
Paul

Hello,
The amount of CO2 needed to be injected in order to reach a certain amount of dissolved CO2 is exactly the same whether using pure RO, a mix of RO/tap, or pure tap.
CO2 does not care about the alkalinity of the water. Only three things determine the solubility or retention of CO2 in water:
Temperature, Pressure, Salinity.

Having said that, it's necessary to understand that in water having a high alkalinity versus RO water, the pH changes that result from injecting the same amount of CO2 will be different. This difference is caused by the higher alkalinity which resists a change in pH. In fact, this is by definition, what alkalinity is. If you add the same amount of acid each to a sample of RO water, RO/tap and to a sample of KH 11 water the same amount of acid will enter the water, obviously, but the ph drop in KH 11 water will be slight. The pH change in the RO/tap water will be moderate and the pH change in the RO water may register as very large.

In our case, the acid that we are adding is a weak acid called Carbonic acid, which is formed when CO2 comes into contact with water. It is unavoidable and is complicated, however, it's only necessary to understand the different behavior of pH when acid comes into contact with the various types of of water.

Also, you cannot really compare your LFS system with your own. Their injection pressure may be high, for example, set by their regulator working pressure. The temperature of their water may be different. The tank may be more tightly covered and may retain more CO2. Their injection method and flow/distribution in the tanks may be different, so it's really difficult to compare tank to tank.

If you cut your tap with 50% RO, then KH and GH would halved, but it is unclear by how much the pH would change.
pH is a logarithmic calculation. KH and GH are linear (proportional) calculations.

In any case, it is unimportant what the pH of the RO/tap mixture is. It only matters what the change in pH is when you add the CO2.
That is the information you look for when trying to determine whether you are adding enough CO2.

If your resulting KH is around 4-6, then you can aim for a 1 unit pH drop. If the KH is higher due to using a greater percentage of tap then the pH drop when adding the same amount of CO2 will be less.

Hope this helps.

Cheers,

 

[Paul195]

Hi

Thanks for the detailed explanation. That explained my question perfectly.

 

[Cor]

If your resulting KH is around 4-6, then you can aim for a 1 unit pH drop. If the KH is higher due to using a greater percentage of tap then the pH drop when adding the same amount of CO2 will be less.

So the 1 point pHdrop method is only accurate at a KH 4-6?

 

[Edvet]

Aye, i used a 1.5-1.6 drop in softer water.

 

[sciencefiction]

Additionally to what was already said, the effect of CO2 on KH is so little, in practice it has no effect whatsoever.

 

[HiNtZ]

Hi Paul
The relationship is simple and complex, depends on
I live in a hard water area and go though 6.5Kg of CO2 a month on my 500l tank. But I do have it at very high [CO2].

Very hard to compare one tank to another esp when talking BPS, mine is insanely high but it works well for me.

Hope that is of some help.

Zeus

I was going to put the graphs etc but I'm sure you have seen them, sometimes less is more

6.5kg A MONTH?? Jesus!

I do 5kg in 6 months with a BPS of 10-12 on my 155L GH6 KH4. Same as you though really, my tank requires massive amounts to get slightly above average levels. Everyone else I know on CO2 seems to get away with 3-4 bps on similar sized tanks.... but their light is way way weaker than mine.

My CO2 also comes on 5 hours before lights on.

 

[Edvet]

I did 10 kg/6 weeks in my big tank

 

[Zeus.]

I was concerned about the amount of CO2 I was using, until Clive informed me that he was using a 22Kg clyinder every 5-6weeks on his 700l tank.

Sent from my Moto G (4) using Tapatalk

 

[Paul195]

The amount of CO2 needed to be injected in order to reach a certain amount of dissolved CO2 is exactly the same whether using pure RO, a mix of RO/tap, or pure tap

In any case, it is unimportant what the pH of the RO/tap mixture is. It only matters what the change in pH is when you add the CO2.

I thought I was clear about this but it turns out I'm not! I understand that the KH will affect how much the PH drops due to the buffering capacity (small swings in high KH, larger swings in low KH).

Are you saying that if you inject at "X" rate no matter what your KH is, then the same amount of CO2 will be dissolved in the water, the only difference being the amount that the PH changes by ? in other words the KH has no bearing on how much CO2 you need to inject, it only affects the amount of PH you need to drop by to achieve optimum levels ?

Does the starting PH of your water before adding any CO2 have any bearing on the amount of CO2 required ? I am keen to try to understand why the amount of CO2 people add varies so much.

Also, you cannot really compare your LFS system with your own. Their injection pressure may be high, for example, set by their regulator working pressure. The temperature of their water may be different. The tank may be more tightly covered and may retain more CO2. Their injection method and flow/distribution in the tanks may be different, so it's really difficult to compare tank to tank.

If I wanted to make my CO2 last longer, am I right in saying the only ways to do this are by lowering the temp of the water, fitting a lid, and ensuring flow and distribution is optimal, and water parameters have no part in this ?

Hoping somebody could clarify for me...

Many thanks

Paul

 

[dw1305]

Hi all,

Does the starting PH of your water before adding any CO2 have any bearing on the amount of CO2 required ?

I'm not a CO2 user (or a chemist), but I think the answer is that a one uit pH drop gives you about 30ppm CO2 what ever the initial pH and dKH readings were.

There is an explanation in <"Question about pressurised...">.

cheers Darrel

 

[Edvet]

I'm not a CO2 user (or a chemist), but I think the answer is that a one uit pH drop gives you about 30ppm CO2 what ever the initial pH and dKH readings were

Not sure about this
1) pH is a logarithmic curve, so ad lower pH you'll need to add more to get the 1 point drop
2) CO2 giving the acid rection (H2CO3) can be buffered by higher carbonate and bicarbonate values, thus needing higher values to get the 1 point drop in "hard"water

 

[ian_m]

Below is the a table (from Barr Report) that shows pH vs kH and resulting/caused by ppm of CO2.

Thus, for example, your initial water has a kH of 4 and pH of 7.6. If you drop the pH by 1 to 6.6, by injecting CO2 the ppm increases to 30ppm.
Similarly for water of 16kH, with initial pH of 8.2, dropping pH to 7.2 the CO2 is similarly 30ppm.
Thus you see 30ppm produces an approximate 1pH drop.

This should be taken "roughly" of course, as the chart below assume pH is solely driven by kH value and CO2 value. In a real tank there are other buffering chemicals other than carbonate and other acids/alkalis present affecting pH values. Generally though I think you get an under read ie 1pH drop caused by less than 30ppm, meaning at least you don't gas your fish. A drop checker can help out here as it is only affected by CO2 ppm and not anything else in the tank water.

 

[dw1305]

Hi all,

1) pH is a logarithmic curve, so as you lower pH you'll need to add more to get the 1 point drop

Ed I'm not sure, it is back to needing a chemist, but think it is because pH is a log10 scale, and if you plot an exponential curve as log10 values you get a straight line.

It is this bit and links (from <"Question about pressurised CO2....">

..... But I think I've got it.

Assuming that the "pH drop of 1" for ~30ppm CO2 is right, it is because it is a buffered system, and pH is a log10 scale. The drop in pH is determined only by the concentration of CO2 in the water.

In the same way a reserve of HCO3- (dKH) will maintain pH at ~pH8 at atmospheric CO2 levels, as you increase the CO2 levels you drive the equilibrium towards H2CO3 and the pH falls.

Assuming you still have a reserve of buffering then the fall in pH may appear to be linear (exponential data plotted as log10 values will form a straight line).

I think everything starts from the assumption that water in equilibrium with atmospheric CO2 levels contains ~3ppm CO2. A drop of 1 pH unit is actually an increase of 10 in the ratio of H+:O-H ions, when you have a pH drop of 1 unit you have:

3ppm x 10 (1 as log10) = 30ppm CO2.

This is from <"The Planted Tank"> This also looks an understandable link <http://institutebishop.org/enviro_11.pdf>.

cheers Darrel

 

[AverageWhiteBloke]

If I wanted to make my CO2 last longer, am I right in saying the only ways to do this are by lowering the temp of the water, fitting a lid, and ensuring flow and distribution is optimal, and water parameters have no part in this ?

Yeah you could do all that, some also say that misting the tank has benefits rather than fully dissolving the co2 in the water. The small bubbles cling to plant leaves and get absorbed directly through the leaf. A lot of people don't like the mist this causes including myself but I tend to have the majority of the injection phase going on while I'm at work so I don't have to look at it. By the time I get in and had my tea and a scrub up the co2 injection is nearly over and I can view the tank as nature intended

The other option is the lighting, I'm guessing that kit you have there has some level of control. The 30ppm figure or 1 drop in PH is like the drop checker, not an absolute but a general rule of thumb figure that people in the planted tank community have decided to be a indication that there should be sufficient co2 available for plants and bordering on being unsafe for fish. You have to use it at some discretion as with the DC it tells you in that particular area of the tank a couple of hours ago there might have been enough co2. Similarly measuring ph of water which should be done in the tank as the gas has already left or leaving if you take out a sample will only tell you what the levels are in the upper rich co2 water level as the gas is just about to escape into the atmosphere of which 90% will.

If you want to conserve gas then controlling the lighting is the key. Having too much light in either duration or intensity will increase the demand for co2. The lights set the pace and you need to try and match that with your injection. It's fair to say that there are many beautiful slow growing plants in the hobby that if not given too much light can get away without having to use added co2 at all. In fact some of these plants prefer you not to accelerate growth too far as the longer the same leaves are in the tank the more susceptible they are to growing algae on them. Faster growing stems etc people tent to trim off the leaves more often before algae gets the chance to coat them.

What I tend to do these days is keep the tank lighting minimal for a while so's not to push the plants too hard and induce deficiency then giving the algae the advantage over weak plants. Add a bit of co2 for a few weeks, the plants will surely love that. If you feel confident then maybe knock the lights up a setting or another half hour with a slight increase in co2. That's why it is difficult to second guess anybody's tank, they all act differently depending on so many factors, even the plant mass changing will affect co2, it might be right when you setup but as the tank fills with plants there will be less flow and more co2 uptake. Simply giving the plants a good clip out can bring things back inline before having to turn that co2 wheel.

I wouldn't get too stuck on the 30ppm, it's not that accurate and maybe your tank doesn't even need it depending on the other factors. Its all about finding what your particular tank needs and trying to maintain stability at that. For instance any time I managed to get a 1 ph drop in my tank the fish didn't like it and gasped at the surface. I love a dense planted tank but not to the point of stressing fish out. I try to aim for a balance in there where flora and fauna are happy and complimenting each other. Excessive lighting can tip the scales and you'll find yourself chasing your tail which takes some of the shine off the hobby IME.

Just to add I wouldn't bother mineralising either, just cut your existing water with RO if you want to soften it down. Mineralising RO is just another tail chaser which will have you chasing more hypothetical numbers.