Earthworms are an inexpensive food for carnivorous and omnivorous fish. You can find earthworms in your back yard, purchase them from a pet store or breed them yourself. Most fish will eat earthworms but depending on the size of your fish you may need to cut the worms up to an appropriate size for the fish to be able to eat. Feeding earthworms to fish is good for their diet but is not recommended as the only item on the menu.
Shake off any loose debris from the worms and wash them with water to remove dirt or pesticides that may be on their skin.
Cook plain oatmeal using water. Do not add additional ingredients to the oatmeal. Allow the oatmeal to cool. Once it has cooled scoop it into a container that is large enough to store the worms.
Place the earthworms into the container filled with oatmeal and leave them there for one day. The earthworms will eat the oatmeal and it will clean their system of dirt or pesticides that could be toxic to your fish. If the oatmeal begins to dry out, add a small amount of room temperature water to moisten it (if an earthworm's skin remains dry for too long it may die).
Remove the earthworms from the oatmeal after 24 hours. Rinse them off with water to remove any oatmeal that may be on their skin. Discard the oatmeal.
Drop a couple of earthworms, or pieces of earthworm, into your fish tank or pond. Watch for a minute or two to ensure that the earthworms are the appropriate size for your fish to eat. If the worms are too large remove them and cut them into smaller pieces. Cutting the earthworms into small pieces with sharp scissors is safer and more convenient than using a knife.
Repeat Steps 1 through 7 every other day or a few times a week. Do not feed earthworms to your fish every day as it is not a balanced diet for the fish.
Dispose of the oatmeal after 24 hours. Do not reuse it for the next batch of worms. Once the oatmeal has dried out it must be remoistened or replaced immediately as the worms need moisture in their environment to live -- if the worms' skin remains dry for an hour or more they may die.