Yesterday, my opaline gourami went erratic when I turned on the aquarium lights (standard 20w flourescent)...at least I didn't notice anything strange before turning the lights on. He started flipping and swirling, and then he would sink to the bottom, appearing to be dead. A little while later, he would managed to right himself and struggle to the top of the aquarium to breathe. He went through this routine two or three times...he was extremely weak and would get pulled toward my penguin 125 power filter (20 g long tank), and the return from the filter would push him down to the bottom. Well after a while, he sunk to the bottom and didn't get back up and died. Early this morning (about 1am CST), one of my zebra danios began exhibiting similar behavior and he too was dead by 3am. I've tested the water: pH 7.4, temp. 82*F, ammo 0, no2 0, no3 0. There are no visibile signs of bacterial or parastitic infection on the fish...no spots or anything like that. I've been searching for possible diseases, but so far, the only thing I've been able to find is possible brain/nervous system damage. Any ideas? I think my last danio may be starting to exhibit symptoms. The other tank inhabitants are 3 gold skirt tetras who have not been affected at all.