Is my aquarium ping around a fish disease?

Okay, some of my fish just die. I got the first fish from a fair, would that might of caused a disease chain? The fish I had gotten is dead, now. but then I got 2 more fish, one of them got sick, but he is still in the tank now swimming, and then one of them (I really think he was hyper.) just jumped out. another bottom feeder died, then one of the first goldfish... now I just have 4 fish in my freshwater aquarium.. (1 Tetra, one silver stripe, and two goldfish) the Tetra got sick when I brought him home but he's doing fine now. What is the cause of the fish just dieing? I never overfed them, or took them out of the tank to watch them bounce, or anything. I'm afraid the tetra will die, it's a ten gallon tank, but can anybody help me, possibly? (And I want to know what type of disease it might be if there is one.)