I posted here about a month ago about a fish kill (20 dead) which I thought might be due to CO2 poisoning - well it just happened again, 15 dead this time. All died within the space of a few hours, doing the circle of death swimming routine. Looked back on my notes on what I had been doing, both times it was right after I cleaned the inside of the glass with a sponge/abrasive pad. Well I just found another one of the pads in the house in an unopened package, Scotch-Brite scrub sponge. I had thought it was just a sponge, but after reading the package there is some sort of bacteria killing agent in the thing, and it states in the fine print 'not for aquarium use'. Wish I had read it before, am pretty sure this is the source of my problem. Live and learn I guess.