O2 vs CO2

[Marty M]

Is it possible to have high CO2 if O2 is normal or high? I sometimes observe rapid gilling but O2 tests normal. I have a huge skimmer and whitewater in the sump from the drains. It's especially noticeable after feeding.

[Randy Holmes-Farley]

In general, there is not a strong relationship between CO2 and O2 in a marine aquarium. Either one can be up or down independent of the other. There is also typically very little CO2 in seawater (as it is mostly HCO3- and other forms). I'm not an expert on fish by any means, but IMO, unless you are adding CO2 artificially (as with a miscalibrated reactor) I don't think it likely that CO2 is a problem unless the pH is very low.

I'd suggest that you get an opinion from someone more experienced in fish behavior and problems.

I'll move this to another forum that might have better responses.

[Wolverine]

Well, I don't know if I can write a "better" response, but I'll do what I can.

I agree with what Randy said about CO2 and O2 being independent in aquaria. I also agree that CO2 shouldn't be a problem unless you have a reactor on the fritz, in which case your pH will be out of wack.

From TRA v.1, page 227: As CO2 dissolves in seawater, it immediately forms carbonic acid (H2CO3), so that little CO2 is actually present in the water. The carbonic acid dissociates into carbonate and bicarbonate ions"
This dissociation releases protons (H+), which is what you're measuring for with pH; more protons means more acidic. This is why the CO2 cannister for a calcium reactor lowers the pH, causing the aragonite to dissolve.

So, in summary, if your pH is OK, I don't think this is a problem.

Have you noticed any fighting between the fish in question with anything else? Really just about any stress can cause the fish to increase their breathing rate. For example, if your fish are fighting over their food, this could explain rapid breathing after you feed.

Dave