You should feed your fish 2 times daily and only what they can consume in 2-5 minutes. Overfeeding fish is the most common mistake fish owners make, the left over food clogs the filter and breaks down into toxins that are harmful to the fish. You also don't want your fish to overeat.
If you feed with pellets, choosing the correct size is very important. If the pellet is too large for the fish they will spit it back out. You want to use a pellet size that the fish can swallow whole. If you have a variety if different size fish in your aquarium, mix different size pellets to ensure that all the fish receive their portion.