All the fish in one of my 10 gallon tanks just died. Ammonia and Nitrite 0, Nitrate 5 ppm, pH 7.4, everything seems normal. I did a water change this afternoon, and I'm guessing that's what did it. I forgot to do a 20% water change last week on the tank, so I'm wondering, is it possible that CLEAN water killed the fish because they were not accustomed to it? I added the water rather quickly because I was in a rush.