Flake foods
Flake foods are the biggest portion of diet in most captive held fishes. Not always the best however. Just be sure its not the only food item. Look t the ingredients(listed from greatest to least). There should be lots of things like shrimp meal, crab meal etc. Depending on your fish, you may want to choose a flake with lots of protein(such as made for guppies) or maybe some more vegetarian based flakes. This all depends on the species of fish. Big fish usually are not fed flake, and picky ones like puffers usually won't accept them either. t is quite difficult to feed bottomfeeders with flake, as it floats, but will sink if you stick it underwater with your fingers. Other than that, most community fish will, and this is probably the weapon you should pull out if you have a community tank. Of course, some fish will only grudgingly take flakefood if they even eat it at all. Obviously don't use this for them.
Pellet foods
These foods are basically what they sound like, little round morsels. Many pellets are made with different fish in mind, such as cichlid pellets. They are usually for big fish(cichlids) or small fish with rather large mouths(bettas and other anabantoids). They often come in different size grades too. Choose one your fish can readily fit into its mouth. Of course, small fish like you wee neon will not be able to eat these(I've had a longfin zebra danio choke on one) and since they float, most bottomfeeders will not be able to get them. This is a good type of food to use for pond fish because it floats, allowing you to watch the little pellet to be helplessly engiulfed by a fish, and it also lets you clean up leftovers. Sinking pellets drop quickly to the bottom to serve fish that are too shy to forage near the top.
Tablets and wafers
These are sinking morsels and some can even be stuck to the tank sides for fish. They are specially designed for bottomfeeders, but most other fish will try to steal(and look like they are carrying dinner plates in there mouths). They come in different types. There's algae wafers for algae eaters, and shrimp tablets for umm..shrimp eaters. Crayfish and ghost shrimp appreciate these too ,and they will often dart out to lug one into there hide and eat it at there leisure. It also works that many fish enjoy picking at these pellets. My montezumae swordtails enjoyed eating wafers mean't for catfish by mobbing them.
Freeze dried foods
These usually consist of some dried daphnia,bloodorms, or brine shrimp. They are expensive, but many fish like them. Works well as variety for fish which eat flake. I particularly like Hikari Bio-pure daphnia as along with full grown adults it also comes with dust fine baby daphnia perfect for feeding a tank of mixed sized fish(such as a colony of livebearers).
Peas and veggies
Many fish(particularly goldfish and also IME American flagfish) enjoy eating peas. The skin should be peeled, and the halves dropped into the tank. Plecos are particularly fond of cucumbers and zuchinni. Boil before serving.
Earthworms
Its good fish bait for a reason. Small fish can eat them chopped, and larger ones can eat them whole, and wriggling. Crayfish enjoy eating these, and watching one reminds me of someone fighting a boa constrictor. If the worms get away, they will "burrow" under the gravel and may die, fouling the water. Don't let that happen. If you have a lot of fish which enjoy worms you may consider setting up a worm bin and culturing redworms-I hope to write up something if my attempt is successful. Blue gularis(Fp. sjoestedti)enjoy redworms but they should be cut in to bite size pieces for them.
I have recently set up a worm bin with newspaper, shredded paper, and some potting mix mixed in. A few pieces of cardboard over the top the keep it dark. Added 1 pound of worms. Also gave them a pear core and a little bit of small pellet fish food. Mosquito larvae
Often, tire ruts, tires themselves abandoned kiddie pools and other standing bodies of stagnated water are homes of mosquito larvae. You could set up a bucket with a banana peel or other organic matter in it as a place of mosquitoes to breed. However the unfortunate mosquito who inserts her egg raft there has just condenmed her offspring to the status of fish food(though that is higher than a pest)! You may remove the egg rafts and raise the larvae inside(easier and less hassle) or you may let them hatch outside, harvesting thme with a net. Method one works better and allows you to control size-but method 2 allows larger wrigglers with less work. For obvious reasons make sure you check on these buckets daily so no egg rafts have a chance to hatch. They can be fed on yeast,breadcrumbs, and the like. If a film forms over the surface of the water, move mosquitoes to a different container. Fish love them, and they make good conditioning food. Bettas, killifish, and livebearers will stuff themselves on this nutritous food. They work best for surface feeders of course. Hatching egg rafts gives you the smallest sized larvae and would be very advantageous if you are raising juveniles.
black worms
. Many stores are now offering blackworms. Keep them cold in the fridge and rinse them out daily to remove and dead worms. A worm keeper works well. If large portions of the worms begin to die it may be best just to throw them all away. You could store them at lower densities in containers and change the water often to avoid having to wash them every day. I'm doing some experiments with culturing blackworms and if it is feasible will post updates.
Bloodworms and glassworms
These are midge larvae, and are a favorite of many fishes. Often mixed into mosquito larvae cultures. Sometimes available live, but I haven't used them live myself.
Daphnia
Daphnia, also known as water fleas, are small crustaceans that live in just about any pond. They prefer fishless ponds, for obviuos reasons. Huge swarms often are seen swimming through the pond, and a net can be used to catch them. Feed immediately to the fish or use to set up a culture. You can put them in a container that receives sunlight. Add a little yeast, and maybe some greenwater and then add the daphnia. An aerator(not tiny bubbles) increases prouduction. They do well in fry tanks as they help keep the water clean, and they produce babies for the fry to eat. There are several different species, each prefering a different temperature. Daphnia moina the least hassle free at the higher end of the range. You will probably be unable to ID them yourself. Daphnia cultures are prone to crashing so keep the population densities low by frequent harvesting and have backups. Most like to keep at least 3 up and running at one time. Many sites have extensively detailed daphnia culture methods and since I haven't found anything too interesting I will leave it at that.
Infusoria
Great for fry that are a bit too small to eat other things, infusoria consists of small organisms in the water. You can just start up a suitable place for them to live and hope that it will be populated. Simply put some water in a jar where it receives sunlight, and add a lettuce leaf. You can boil barley straw and let the water cool and use that too. After some time, you should be able to feed some of it to the fish simply by adding the water which the infusoria are in. The water should turn green-but should not smell.
Microworms
Microworms are tiny netamodes that are easy to culture. A simple margarine tub with a recipe for microworms(should be able to find one that suits you)..and some starter sets off the worst(best) population explosion you've ever seen in your house! Simply use oatmeal and add some yeast. They will then reproduce at a wonderful rate and provide food for fry or even small adult fish like Heterandria formosa. I find them excellent for colony tanks.
Scuds(gammarus or Hyalella)
Tiny shrimplike animals commonly known as scuds. I have been culturing them in a large critter keeper and add paper towels, plant clippings, dead fish, or anything else that comes along. It seems to all dissapear into the black hole. You may find these in ponds or amongst the roots of plants in slow moving water. Several dozen is all you need to start a culture. They reproduce slowly and thus cannot be depended on as a staple for the fish unless you set up very large cultures. The adults are anywhere from 1-2cm and thus scuds are not suitable for tiny fish. The babies are only several mm long but I haven't found a way to seperate them from the adults. Scuds are hard shelled and swim remarkably fast. Sunfish, small cichlids, and other similar fish love them. If they escape they may establish a population in the tank or at least serve as scavengers for a while before being found and eaten. They have been known to feed on fish eggs so are best kept out of breeding tanks.