what kind of fish do you have? some fish... like goldfish contrary to popular belief.... should not be fed floating flake foods, as it causes them to eat air along with the food which combined with goldfishes' over-inbred delicate digestive systems can be fatal. Also many fish don't need to eat very much. Make sure you are not overfeeding which can also be deadly due to a variety of things. I reccommend going to your local aquarium store. A store that specializes in fish, not one of these pet emporiums like p*tco or p*tsmart as their lack of knowledge and inability to educate their customers is borderline criminal. If you don't live near any reputable aquarium stores try an online search for more specific information concerning the kind of fish you have and it's care. There are many online chat rooms with a lot of knowledgable aquariaists that are a lot more active than this forum. Your local library is also a good resource.
If you want more help from me tell me more about your pet. what breed? age? size? what is it's tank like? How many fish are in it? filtration? bubbles? gravel? What are the ph, nitrite, nitrate, amonia levels? What exactly are you feeding? Explain how often and how much water you change out. Do you use a water conditioner/what type? I'm no pro... but I have a bit of experience.
One thing, if your fish isn't eating you don't want to keep adding food as when food decays in the water it releases toxic gases that can kill your pet.