A few more details please.
What size is the tank?
What fish are in the tank? How big are they?
How often do you change the tank water, and how much do you change?
How much do you feed and how often?
What foods do you feed?
Do you own a test kit? If so what is your ammonia, nitrite and nitrate levels?
Have you added any new fish, plants, rocks or wood recently?
Did you clean/quarantine any new additions?
I'm not a vet so I can't help other than recommending more water changes (when in doubt, do a water change). But if you can answer all the questions I've asked people will be able to assist you easier. The more details you can give, the better the advice you'll get.
Good luck.