I think perhaps we misunderstood eachother. I'm trying to understand your point and see if I can help here.
Originally Posted by KevinVA
If we simplify things a lot we get CO2 from the fish which the plants take together with some light. They use the C to build cells and return the O2 to the water.
During the night the plants do indeed return some Co2 and don't produce O2 but that's not going to be the same amount as they took during the day otherwise they would not be able to grow.
Now your point that the O2 in the water is used during the night and not supplemented is entirely true.
The point I was trying to make is that a casual reader might get the impression that a densely planted tank means the fish will suffocate during the night. If that was so then the densely planted jungle tanks some people keep would not be able to support life and that's simply not so.
18 fish of serpae tetra size in a 75 gallon is not exactly dense stocking.
So.. my point was that O2 levels during the night might play a role but perhaps there's something else at work here as well.
As to bubblers... I know from the aquascaping crowd that simply producing big bubbles doesn't lead to much gas absorption in the case of Co2. I don't see how that would be different for O2. (and you're not just pumping in pure o2, 4/5 of the bubbles in nitrogen).
Surface agitation, either from a hob filter, a spraybar or even a wavemaker, seems to trap a lot more O2 in the water.
I've tested this in the past. A spraybar slightly over the water level leads to a lot of tiny, tiny bubbles that stay in the water a lot longer than the bigger bubbles created by an airstone and pump.