Tropical Fish Keeping banner

Can Anyone Explain The Carbon Cycle In An Aquarium?

16K views 21 replies 5 participants last post by  Byron 
#1 ·
I know the Carbon Cycle is important in aquaria because of the buffer and the CO2 level in the water, but I'm drawing a complete blank. I googled it and really could'nt find anything.

I would love to see chemical equations, if possible. Maybe if you know a page with a good explanaton you could just point me to it.

Thank you. ;-)
 
#2 ·
#3 ·
Hi Byron. i read your article on water hardness and pH, and am duly enlightened, at least partially. Thank you!

I am looking for the equilibrium between the buffer and the water. It would be clearer if I could see the equation - I took Chem 101, which I liked a great deal. But I suppose an explanation would be fine.

Thanks.
Steve ;-)
 
#4 ·
Hi Byron. i read your article on water hardness and pH, and am duly enlightened, at least partially. Thank you!

I am looking for the equilibrium between the buffer and the water. It would be clearer if I could see the equation - I took Chem 101, which I liked a great deal. But I suppose an explanation would be fine.

Thanks.
Steve ;-)
You'll have to explain this a bit more so I can be sure of the question. Presumably you want to buffer the pH in the water, with a suitable KH?
 
#5 ·
My Double Light Fixture With Daylight Bulbs On A 10 Gal.

Btw, I covered one of the big diameter fluorescent bulbs with aluminum foil (can't only have one going, unfortunately). It looks good, still a little bright, but I think usable. I think I will buy a glass top for the evaporation.

Steve ;-)
 
#6 ·
Byron, sorrry if I'm not being clear. I don't want to do anything with the information. I just wanted to know what those carbonates and bicarbonates are doing and how the buffer works chemically. I understand that this is part of the Carbon Cycle, or at least I would think so. I'm interested in these things. Sorry if i'm asking a weird thing or not speaking in a way that is clear to you.

Thanks,
Steve ;-)
 
#7 ·
Byron, sorrry if I'm not being clear. I don't want to do anything with the information. I just wanted to know what those carbonates and bicarbonates are doing and how the buffer works chemically. I understand that this is part of the Carbon Cycle, or at least I would think so. I'm interested in these things. Sorry if i'm asking a weird thing or not speaking in a way that is clear to you.

Thanks,
Steve ;-)
As a chem student you may well know more about this than I do.:) But I'll try to explain what I understand from my aquarist's perspective.

The carbonate and bicarbonate ions in the water do the buffering. These are related to minerals, primarily calcium and magnesium but a few others can also be present. As carbonic acid is produced through the biological processes in the aquarium they are taken up by the carbonates, thus maintaining a steady and stable pH. The more carbonates (= higher KH) the more they can do this. Without carbonates, as in very soft water, the production of carbonic acid will lower the pH because there are no carbonates/bicarbonates to take it up.

Live plants are a factor in this process due to their impact on the amount of dissolved carbon, CO2, in the water.

Byron.
 
#9 ·
You mean this?




Those two above ones should help explain how it works in the aquarium. Though the solid CaCO3 comes from ground water not an actual source in the tank unless you have limestone or crushed coral in the tank. Its one of the reasons you do water changes. As the cycle does slowly consume buffering capacity. In the aquarium though the cycle is very simple for the most part. Planted tanks are a bit more complex. If you compare the carbon cycle in a actual lake its MUCH MUCH more complicated. This is about the best image I could find without getting into biochemistry too much.

 
#13 ·
@Mikaila31:

Thank you very much for the equations. The lake is complicated, isn't it? I copied what ended up being the whole thread, unfortunately, and I'll be looking at the two upper pictures until I understand them. Pretty straightforward.

Thanks much.
Steve ;-)
 
#14 ·
Yes they are straightforward when simplified like that. You have carbonates, bicarbonate, and carbonic acid as the 3 main forms of carbon. They dissociate between one another based on the pH which favors equilibrium towards a certain form.

That lake diagram is pretty simplified and only goes up to plants and algae. Its missing any trophic level above that. Plants are made partially of carbon and things eat them, those hetertrophs use respiration and produce CO2 while breaking down the plant or algae matter. They also assimilate some of it. They in turn may be consumed. What consumes them may not be considered to be part of the aquatic ecosystem my most. Say a eagle catching a fish. Thats basically removing carbon(as well as other stuff) from the aquatic system. Like wise trees often fall over into lakes introducing carbon. The cycle very quickly gets very complicated and thats simply from a biological stand point. I am a chem minor and the chemistry part of it is simply overwhelming. A simple arrow on there like the one photosynthesis step is in truth a pathway of 8-10 chemical reactions. The rest are based on cellular metabolisms which are just as complicated. That one figure simplifies 100s of reactions. That how chemistry works. Its cool to learn but the more you learn, the more you realize there is and the less you feel you know;-).
 
#15 ·
My Water Params Have Changed A Lot!!

I have no clue as to what's going on in my 10 gallon tank. pH is dropping from 7.3 to 6.8 or .6 and nitrates have gone from 10 to 5.

The buffer may have been exhausted by CO2. I also bought 3 plants, Java Moss, some kind that looks like oriental whatever, and one kind with little leaves and no bulb. Insignificant, I would say.

Please someone enlighen me as to what's going on!

Steve
 
#16 ·
If you are injecting CO2 then yes it is going to cause a pH drop. If you are not injecting CO2 then the drop is not going to be due to CO2. How much depends on the buffers present in your water. You would have to test GH and KH to really know that. If they are on the low end then that will make your pH easy to fluctuate. Your buffer doesn't normally get exhausted unless it is low to begin with. The buffer is usually dissolved CaCO3 along with some magnesium. Its basically an acid-base equilibrium. The carbonate and bicarbonate are basic. When an acid is added the usually bind it and both are 'neutralized'. The pH will fluctuate somewhat before all the buffers are gone. If the buffers will actually all exhausted pH would hop around wildly.

The plants can explain the drop in Nitrate, but also realized aquarium nitrate tests are very inaccurate.
 
#17 ·
Thanks Mikhaila,

A good explanation. I just wish I knew where the acid is coming from in my tank. I used Wardley Bullseye 7.0 before I heard about the phosphate in this stuff, and I haven't done any water changes. Just small top offs with tap water. No ingredients on the label.

Steve
 
#18 ·
Thanks Mikhaila,

A good explanation. I just wish I knew where the acid is coming from in my tank. I used Wardley Bullseye 7.0 before I heard about the phosphate in this stuff, and I haven't done any water changes. Just small top offs with tap water. No ingredients on the label.

Steve
This is easily answered, acid occurs naturally from the biology. CO2 comes from fish and plant respiration, from bacteria breaking down organics, etc. Mikaila well explained the buffering side. The pH in my tanks is down at 5 since I have no buffers (zero KH) in the tap water, or if I buffer two tanks with aragonite the pH stays around 6.4 (in the 115g) and 6.8 (in the 90g) by design.

As Mikaila said, we need to know the GH and especially for this the KH/Alkalinity of your source water, and tank water if different. The source water will basically remain unless something is specifically targeting it in the tank, like calcareous substances or something.
 
#19 ·
Thanks for the explanation, Byron. From the most recent MWRA report, Distribution system sample pH ranged from 9.1 to 9.6 and alkalinity ranged from 40 to 42 mg/L. And...

Total Dissolved Solids 35.0

I hope Total Dissolved Solids can help you with GH. The report explains that it is GH.
 
#20 ·
Thanks for the explanation, Byron. From the most recent MWRA report, Distribution system sample pH ranged from 9.1 to 9.6 and alkalinity ranged from 40 to 42 mg/L. And...

Total Dissolved Solids 35.0

I hope Total Dissolved Solids can help you with GH. The report explains that it is GH.
That's a fairly low KH, 40 mg/l is roughly equal to ppm, so 40ppm which is approximately 2.2 dKH. I would myself expect the pH to lower as the buffering will be minimal, but Mikaila can confirm or correct this.

So this explains where the acid comes from, presumably?

You also have very soft water, around 2 dGH though this is their TDS and I've no idea how much of this is calcium/magnesium though I would suspect most of it is.

Earlier you mentioned using chemicals preparations to adjust pH, I would counsel against this. It is better to let nature take its course. Forgotten what fish species, but soft water fish will have no issues with just letting it go naturally, although one doesn't want it too low for some. If buffering is necessary, the method I use is safest, in my case just a bout 1.5 tablespoons of aragonite in the filter buffers the 115g and similarly the 90g as mentioned previously. And this lasts years.
 
#22 ·
Thanks a lot Byron and Mikhaila. If you want to look at an actual report, including all of the dissolved solids, please go to MWRA - All Water Quality Tests. Thanks for all your help.

Steve
They have hardness average of 15-20 mg/l (ppm) which is very soft, about 1 dGH. My tap water is half that, at about 0.6 dGH. And adding Akalinity to raise pH makes sense. Vancouver does this with some sort of ash.

You will need to raise the GH for plants, it should be no less than 4 dGH in order to provide sufficient calcium, magnesium and potassium. I use Seachem's Equilibrium for this. The aragonite does not add sufficient GH without sending the pH through the roof.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top