I have had my 55g tank for about 4 years but am not an expert. I usually have gold fish and currently have a small fantail and a several zebra, which I have had for over a year. The amonia level is fine but don't have a full on water testing kit. I just purchased a fairly large oranda on Friday, and aclimated him really good. The guy said he liked to swim upside down and saw him doing it.....no problem. But after about an hour being home today I notice he was not turning over and his mouth wasn't moving and then he was dead. The other fish all seem fine, the zebras have been hanging out on the top(not sure if this means anything). I did do some vacumming, new water and new filters this weekend. Could it have been that or could the new fish have been sick???