This was not a tank of mine, but I'm so stumped on this one and was wondering if anyone could give me some ideas. I'm the aquatics specialist at my local Petco, and we had someone that came in and bought a 29 gal kit (heater, filter, water conditioner). They set up the tank properly (rules out new tank syndrome) and the bio filter was set up properly. They came in and bought (if i'm remembering correctly) a pleco, some white skirt tetras, and mollies first. All dead in an hour. Came back and bought 3 more fish (not sure what), all dead in an hour. Came back two more times and the same thing, even killed a single feeder goldfish in less than an hour. Tested his water every time he came in, everything was perfect. From what I've seen, there's nothing he was doing wrong that could have killed all those fish so quickly. He rinsed everything with hot water before placing it in the tank, and didn't use anything to clean that could have harmed the fish. Does anyone have any ideas as to what was causing this? He returned the tank and is starting over completely, but I'd like to know what could possibly have caused all the carnage.