all this talk about CO2 and O2 and N2 in solution. Here's a new
question: do fishes contribute to the nitrogen compounds in
solution (ammonia, nitrates etc.) by consuming gaseous nitrogen
and converting it somehow? We know that under certain circumstances
bacteria will convert nitrogen compounds back to gaseous nitrogen
which would result in a net loss of nitrogen in the system (ignoring
additions of fertilizers, food and new aquatic creatures)