I bought two little sharks yesterday at Petco.Theyw ere both fine last night and this morning. Then I noticed one was swimming upside down.I have them in a 5 gallon tank and I know he is dying.. I don't know what to do? This is my first fish. There is a filter that I ran for a week before putting any fish in.The heater is set at 75 f. The other one is doing well..Any ideas how to save him?