I haven't posted in a while but have been reading. I had a betta named Swimmie, and a gold mystery snail named Jesse Owens, in a 5.5-gallon tank with heater and filter since June 3. Both were doing awesome. I stupidly added 2 otos on Saturday (10/14). By Monday, everyone in the tank was dead. I tested the water before adding the otos, after adding them, and after everyone died. Nothing was wrong. There was no indication of disease or anything. I had the otos quarantined prior to adding them. When I purchased them they looked good. Stomachs were full and round with pink on their undersides. Why did I add them? Did adding them stress the betta out and he died. The aquarium store refunded my money for the otos and will replace my betta for free. I think they feel as bad about it as I do. Any thoughts as to what I did wrong?