Can you give us some more information to work with please?
Tank size - volume and dimensions
What fish are in the tank - species and numbers
What are the readings for pH, ammonia, nitrite and nitrate?
What is the GH of your water? If you don't know, look on your water provider's website.
How often do you do a water change, and how much do you change each time?
What type of food do you feed (eg flake, pellets, live food etc)
What chemicals do you add to the tank - presumably water conditioner/dechlorinator but anything else?
The more info you can give, the better it will help to work out what the problem is.