Infusoria
This is probably one of the easiest water live foods to create and culture. All you really need is a jar and some lettuce. But there are a few ways to make it more effective.
Get a jar, make sure it doesn’t have anything nasty in it, clean it out. Fill it with dechlorinated water, also preferably a little bit of pond water or aquarium water, chuck in some strips of lettuce (I use cos, because my guinea pigs eat it). Close the lid and leave it for a week/two weeks. The inside of the jar will be occluded and it will be a a mixture of infusoria and green waste.
You can feed this directly or you can store the mixture in a larger container, I keep mine in a small tank, it makes it easier to dispense to tanks with a turkey baster.
The larger container will also occasionally need more lettuce and water from the jars as a staple food to keep the culture going.