Need help with my tank

Hey guys. I need help figuring out a problem. I have a 55 gallon tank with
4 decent size piranha in it. About an inch of gravel, air hose, I replace
the filter pads about once a month or so. Problem is, the water is very
blurry or cloudy. I have tried various brands of those clear water drops,
but they just don't do anything. Can anyone tell me if I am missing
something or doing something wrong? It's gettign very frustrating to have a
large tank with nice fish but you can barely see from one end of the tank
to the other. Please tell me anything I have to do, check, or whatever to
fix this problem. Thanks in advance!