Ok , so I bought this active Pygmy angelfish about a week ago and it seemed to be healthy it was swimming getting along with tank mates eating ect. And then when I was working on my other tank and I checked on him he was fine then when I came back less than 1 min later he was just dead on the floor of the tank . Nothing seemed to be wrong I recently did a water change and tested water nothing seemed abnormal. Anyone have any surest ions on what could have happened??? ( he was 1inch or smaller)