You need to research some of the basics of fishkeeping I think. Let me guess, you got this tank for christmas and set it up and added fish the same day?
Let's at least get you some good advice for now. What kind of fish are in your tank? How big is it?
As you are cycling with fish you need to learn more about "fishy cycling" (going through a cycle with fish in the tank) I don't do it personally so I can't speak from experience. I like to cycle a tank before adding fish... There are quite a few threads and articles which address the subject, for those of us giving advice here, let's at least make sure your tank has been set up with a successful bioload, otherwise you're just going to have more problems down the road.