Tubifex worms can be bought from most fish shops and they are eaten by most if not all small fish. Tubifex worms are grow up in dirty and bacteria laden mass that comes together with the worms so it is essential to rinse them under running water before feeding them. Otherwise, you will be putting a lot of these things into your water!
Some people stay away from tubifex worms because they are thought have a high chance of bringing disease to fish that eats them.
There are also freeze-dried sterilised tubifex worms available in cans in the form of small compressed cubes. When feeding these, they are very light so you can give these cubes a light squeeze in the water to help them take in water and sink. (unlike freeze-dried bloodworms, freeze dried tubifex are readily accepted by fish)