Red worms found in ponds are often called bloodworms because of their color. They are usually about 5 inches or shorter.
They are usually the larvae of midges or the Chironomus species but can also be actual tiny worms from the Polychaeta family.
The bloodworms are sucked up by the pond filter's intake valve. The red worms are not small enough to go through the filtration media such as charcoal, zeolite or cotton fabric, according to "Freshwater Aquariums for Dummies".
Bloodworms eat decaying plants, dead fish and any food that the pond fish missed. Too much decayed matter produces ammonia.
In this way, bloodworms help to keep ammonia levels in the pond low.