The fish food you feed your fish will have a huge impact on your fish. Good food will give you healthier more colorful fish. Bad food will give you fish that are less colorful and that are more likely to attract diseases. Good food will also make your fish grow faster and make them more likely to spawn. It is worth the money to spend a little extra money to give your fish a high quality food. The fish will reward you for it.
Flake food is very easy to use and availability in pet stores as well as many grocery stores. This is the fish food that most people feed their fish. Flake food is unfortunately not very good. A high quality flake food will keep your fish healthy but it will not show you its best color if you feed them exclusively flake food. Many more sensitive fish will not accept flake food at all.
There are some very good and some not so good pellets on the market. A high quality pellets can be a very good food to feed your fish. It is easy to use just like flake food but is usually higher quality. Pellets can be a little more expensive then flake food but the difference is very small and it is worth using a high quality pellets (granule) instead of a flake food.
There are pellets designed for different types of fish of different species.
I recommend that you feed you fish some other food as well but pellets can be a good base.
Frozen fish food
Frozen food is very good food and an easy way to give the fish a diet very similar to what they would eat in the nature. You simply cut away a small piece of food from a frozen block and feed it to your fish. There are a large selection of different frozen food available ranging from mosquito larvae to different types of shrimp etc. Almost all fish love frozen food. The drawback is that it can be expensive to use and that it can be a little bit of a hassle to have to go to the freezer each time you are going to feed your fish. Some people find frozen food dirty to use.
Live food is the best food you can feed your fish but it is unfortunately very hard to be able to offer your fish a variety of live food. It takes space and effort to breed live food. This something that only the most dedicated fish keepers will be willing to do. Some fish stores will sell live food. This live food can be somewhat expensive but it can despite this be worth to buy some very now and then as a treat for your fish. You will see how much they love it.