Fish gasping at the surface isn't necessarily a sign of low oxygen content. I didn't read the other post you refer to but I'm assuming that you changed all the gravel to sand at once. You might have seriously impired the biological action of the bacterial cultures in the tank by removing most of them with the gravel. The tank could be re-cycling, suffering ammonia and/or nitrite spikes. Have you tested the water for the presence of these chemicals? If not then do it as a check. If you have, and the amount of both are zero (the only readings that indicate a cycled tank), then I'd guess that the sand you used might be poisoning the fish in some way. Did you use aquarium grade sand or washed out builders sand?
Either way you need to remove the toxins from the water asap! I'd suggest 30-40% water changes daily until the toxins revert to zero. Do your tests on the water to check this.
Unless your tank is overstocked or you have fish that love oxygen rich water I cannot see why all of a sudden your fish cannot breath due to oxygen depetion. I think your problem is due to something extra in the water rather than something missing!
Although plants add oxygen to the water you'd need large quantities of specific species of plants to make a difference. A good surface current will be far more benificial to the oxygen content than plants (in my experience). Get your airstone back in and get the water surface moving. By the way, airstones add oxygen by causing a current at the surface so improving gaseous exchange with the surrounding atmosphere. The bubbles themselves do not directly add much at all!
WK
P.S. Just checked your last few posts. Is the sand you used coral sand? What fish do you have and what's the pH like now. Your fish could be suffering in the wrong pH. It'll kill them!
which is good
Anyway, only way I know of to lower your nitrites is through water changes, unless you're able to add a mature filter from another tank. That would help, too. Maybe other people have other suggestions?
