Well, I discovered in the interim that every single snail in my tank suddenly died, and my otherwise healthy looking oto also took the gas, seemingly all at once. I tested the water, Ammonia 1.0, Nitrite 0, Nitrate 0, but a very low ph reading below 6.0. All this was before I found the dead oto. I removed the oto, then put 6 tsp amquel in and turned on the airstone. Two days later, all rasboras look normal - no white tips on fins! I have been turning on the airstone at night when lights are off, and they all have their appetites back and seem normal. However, the tiny whitish spot on my betta's pectoral fin has returned. It seems to come and go. Did my tank have a pH crash, and could this have killed the snails and oto?
Incidentally, the tank (60 gal) has been running since July. I didn't do a formal fishless cycle. I used Biospyra and put fish in roughly the same time and they have been healthy for 4 months before this. However, I have never had a nitrate reading above zero. This probably indicates an imbalance. Being a newbie, I don't yet have enough knowledge to know what I did wrong. I do weekly 35% water changes, but I have a low bioload. Its possible I may be overfeeding. Can anyone comment on this and give advice?