Welcome to AA!
First we need a lot more info to be able to help you and your fish.
What size tank?
What kind of fish?
How many fish altogether and how many?
How long has the tank been set up?
Is the tank cycled?
What is the ammonia, nitrite, nitrate and pH?
When was the last water change? How much?
What is happening to the fish?
If you don't understand any of my questions please ask!