I have received many an alarmed email from clients all over the country, all concerning the same thing: a species of small red worms that like to take up residence in filter boxes during the warm season. What are these things? Are they koi parasites? Are they harmful to my fish or my plants? Are they harmful to me? Should I get rid of them? How can I make them leave? One client even freaked out and put their filter material in the microwave oven to nuke the little beasts to be sure they were all killed. (Bet that smelled good!)
Fortunately, there is no need to panic. They are not fish parasites, they are not harmful to your plants, and they are not harmful to you.
They have a name, albeit a rather unsavory one – sewage worms. Sewage worms thrive wherever there is a plentitude of organic waste; in this case, a pond filter. Typically, they are found in waste water treatment plants, digesting aquatic waste matter as an earthworm digests solid waste matter. If you have these little guys in your filter, it is probably an indication that there is too much organic debris on the material. In this way, they are actually beneficial to your system because they are getting rid of excessive waste. However, if you find them disturbing, a good hosing-down of the filter material will dispose of them; or, simply wash them separate from the organic debris and dump them in the pond where they can make a tasty, high-protein treat for your fish. To discourage re-infestation, make sure your filter is cleaned regularly. Like stray cats, if you do not feed them, they will not come.
Finally, never nuke (microwave) your filter material! Besides killing the red worms, it will also kill the beneficial bacteria that make the filter effective.