You have probably discovered that in addition to the large variety of fish and invertebrates available for sale, there is also a huge variety of fish food available.
It is sometimes difficult to choose what is best for your fish or invertebrate animals.
A good starting point is to ask what the animal was eating before you bought it. At least you will have something that it eats. Then, do some research to find out what is the best diet for your fish or invertebrates. Ask the most knowledgeable fish person at your pet shop, or check in a book. Sometimes the internet can be a good source of info, but be careful to check more than one site, just to be sure.
A good tip to help in fish feeding, and in maintaining a healthy aquarium, is to never over feed your aquarium. If you have say 8 - 2 inch fish, and they eat flakes, just add about 16 to 20 bite size flakes at a time. Makes sure that all of the food gets eaten, and does not end up on the bottom of the tank, or trapped in the filter. You can always add a little bit more food, but it is harder to take it out, once it is put in. Overfeeding an aquarium is one of the biggest sources of excess ammonia, and cloudy, dirty water. A fish's stomach is not very large, so don't feed them a lot at one time. The average fish should eat 2 or 3 times per day, but just not a lot of food. Keep your fish solid looking, but they don't have to be bulging at the sides every time they eat.
Carnivorous fish are a little different, and usually they are fed once every day, or even every other day. It is best to not add all of the feeder fish to a tank at one time. Just feed the proper amount per feeding. Keep your feeder fish in a separate tank, so you won't overcrowd your display tank each time you feed.
There are automatic fish feeders that will dispense flake and small pelleted food. These are handy for vacations. There are also 3 day, 7 day, and 10 day vacation food that you can just add to your tank when you leave for vacation. You can do this with flake eaters, catfish, and other not so picky eaters. Carnivores won't eat this food.
When you have finally gotten your aquarium balanced, with just the right mix of fish, you will want to make sure that your water is always in good shape. A water test kit is a good tool to help you make sure that your water is always good. A good freshwater test kit will test for Ammonia , which is the most poisonous thing that could build up in an aquarium. Next, it will test for Nitrite, which is second most poisonous. Next it has a test for Nitrate, which is not very poisonous, except for very sensitive fish.. Next, most complete test kits will have a high range and a low range PH kit for freshwater. PH will tell you how alkaline or acid your water is, and this is good to know. Marine test kits are a little different, but essentially the same. You can purchase any of the test kits separate, and you can even get kits to test for hardness, copper and a few other things too.