You want a cheap food for fish. Does the price of free sound good? One of the major sources of fish food in the world is mosquito larvae. Also you need is a small container with a few inches of water. If the water is a bit green so much the better. The mosquitoes will lay their eggs which hatch in a few days. They can take up to three weeks to pupate and emerge as adult mosquitoes. The female will need a source of blood to have the energy and nutrition to produce the eggs. The males drink nectar from flowers. This will only work in warm weather. You take the water and pour it through a brine shrimp net that you can purchase here on eBay or you local fish store. You take the net to your aquarium and release the larvae into the aquarium. They will be all eaten in a few minutes. Water that ripples push the larvae away from the surface of the water. They need to breathe air. they have a small tube that acts as a snorkel. They feed by filtering algae and micro-organism from the water. It is a good idea to inspect your yard for possible places of standing water. Examples: empty cans, plastic containers, old tires, and anything that can hold water.
Why use mosquito as a live food? They are some of the more difficult fish that need live food to breed properly. Most species of Killifish will not normally eat flake food. They are many livebearers that love insect larvae as food. The live food excites them into eating. Even vegetarian fish need some protein in their diet for proper nutrition.
If you are going to raise mosquitoes on purpose, please be kind to your neighbors. Do not allow the larvae to reach maturity. You must strain all the water at least once a week. Remember in very hot weather the larvae grow very rapidly. If you cannot do every week this keep the container empty of water or do not bother raising mosquito larvae. Mosquitoes can be carriers of several diseases including West Nile Virus which can make people sick and has killed thousands (if not hundred of thousands) of birds in North America.