Tetra fish keep dying, whats the problem?

so i had 2 fish and one of them freaked out and was like running into the rocks in the bottom of the tank and like darting everywhere.. its one of those small, simple tetra fish.. and then it just like went upside down and died... this has happened another time before and so after all those fish died i completley sanitized the tank with really hot water... but i dont know if its some kind of disease thats making them die. so whats the problem?