Why you should culture live food
Most fish prefer live foods, and all fish will live healthier, happier lives if processed flakes and pellets are not their only source of nutrients. Your aquarium fish will develop brighter and more vibrant colors, and more interesting behaviors, if they have a regular and varied supply of fresh, live foods. Many live foods also give your fish a chance to exercise their hunting drive because the little creatures will try to evade being eaten. Packaged fish food such as flakes and pellets are highly processed and usually contain ingredients that are not natural for fish, such as wheat and other fillers.
A live food culture is not only good for your fish, but better for mother nature. No factory is needed, just some spare jars or a spare aquarium and some natural food sources such as green water or decaying vegetation. Rather than supporting factories and supply chains, you'll be developing a miniature food chain in your own home.
Culturing live food is also a valuable learning experience, and allows aquarists to develop a closer bond with their fish and with mother nature. It is an extremely rewarding experience to watch your fish chase down and eat little water bugs that grew up in your own home eating leftover debris.
What to culture
There are many live food options available to home aquarists. Some, such as brine shrimp, require regular hatching and you need to purchase a steady supply of eggs. Others, like daphnia and amphipods (AKA scuds) can breed indefinitely in a closed system. Choosing an assortment of different food sources can make your supply more stable and your systems more efficient, and provide a more interesting lifestyle and complete nutritional balance for your fish.
What you should choose to culture also depends on your goals and interests, as well as the species and age of the fish that will be eating them. Availability also comes into play. You can start many cultures from water samples found near your home, or purchase starter cultures or eggs online or from local retailors. Some cultures will even spontaneously develop in your aquarium from eggs or live samples brought in accidentally with fish, plants, and substrate.
What fish prefer
Pond Snails are one of the easiest and most common live cultures, usually considered a pest because they show up un-invited in nearly every established home aquarium. Pea Puffers consider them a delicacy and many loaches enjoy snacking on them. If you have puffers in one tank and a planted community aquarium in another tank you can simply let the pond snails grow and breed in the community tank, and regularly harvest them to feed to your puffer fish.
Amphipods (AKA Scuds) come in many sizes and variations, and most fish will eat them. The larger ones make an ideal food for adult fish up to 6 inches long, while smaller species or baby scuds can be used to feed fish fry, or small fish such as neon tetras.
Daphnia and Moina are two closely related filter feeders and represent a keystone species in many natural food chains. They are typically smaller than amphipods but also come in a range of sizes. Because they are a cornerstone species, most fish species readily recognize them as a prime food source and truly enjoy them. They are also extremely nutritious and easy to grow. Even the largest species of daphnia (Daphnia Magna) are suitable food sources for small fish, but for very small fish or baby fish fry, you will want to stick to the baby daphnia or culture a smaller species.
Ostracods (AKA Seed Shrimp) are very common in natural systems and eaten by many species. Their tough outer shell and lower nutrition content make them dangerous to feed in large quantities. Many fish, such as betta fish, will engorge themselves on these little water bugs and may develop digestive issues or blockages leading to disease and even death, so just be sure to feed them in smaller quantities and watch your fish eat them to make sure their stomachs aren't budging.
Mosquito Larva don't need to be actively cultured. In most parts of the world during spring and summer, any small stagnant body of water left outside with leaf litter or other decaying plant matter will end up swarming with these larva. It's a good idea to prevent them from hatching into adult mosquitos by dumping out the stagnant water, but before you do this go ahead and collect some with a fine net and feed them to your fish! Most freshwater aquarium fish absolutely love these little buggers and will consume a large harvest very quickly. They are highly nutritious and easy to digest, and your fish will thank you for the fun snack.