Greetings, fish lovers.

I have been using flourish excell in my 55g, and while it seems to have wiped out my cabomba and anacharis, it is *great* for my amazon swords.

Now, it seems to me that a good tank is all about re-creating nature in a semi-closed system. Everything that happens in the tank--food, aeration, filtration, water changes, et al--simulates, recreates, or replaces something that happens in nature. The challenge of making that working mini-ecosystem is a big part of what I love about fish keeping.

So, in dreaming about some new and interesting ideas for biotope tanks, I have been pondering this whole CO2 thing for a while. What is the cognate of CO2 augmentation is in nature? Is it natural aeration (and if so, why doesn't that work in the aquarium?), fish respiration (ditto), some decomposition process, or what?

I like to know the theory behind what I'm doing, instead of just paint-by-numbers. It drives me crazy sometimes. Anyway, thanks in advance for the insights.