Hey Robin....havent posted in awhile, but I always come to you for advice. Thanks for the site!
I live in Ohio....have had our 600-700 gallon pond for 5 years now and truly love it, but boy have we had a tough year with it this year. Lost a lot of fish and have had an algae battle all spring and summer long.
Anyway, recently we began to winterize it and upon taking the pump out to clean the filters I noticed what looks like little baby earth worms(reddish in color) in the filters. There are millions of them.
Could this be a parasite that has caused the demise of so many of my beloved fish this year. The water tests fine, now it is crystal clear, but for 3-4 weeks I kept losing 1 or 2 fish daily. However, 4 have suvived. any suggestions?????? Thanks as always....Hayley

How big are they? It's possible they are blood worms. If they are then they are completly harmless to your fishies. I'm pretty sure blood worms are some type of fly larvae.

Thanks for the help they are about and inch in length...Thanks Again

I don't know what those are but when I clean my filters they are full of them too.
I don't think they would be harmful?? I have never lost a single fish (knock on wood) ;)

Yep, sounds like bloodworms. Bloodworms are midge larvae, and they wont hurt the fish at all. Many people (including me) buy them to feed to their fish. (fishies LOVE them)

I'm sorry that you've had so many fish losses and problems. I don't think the animals in the filter are at fault.

The general rule for parasites is that they don't survive long without their hosts. If they're in the filter or driving around the pond, they're probably not parasites. If the "worms" are bright red and have tiny legs on closer inspection, they are probably bloodworms (a type of midge larvae; midges are gnats). They could also be caddisfly larvae. It's also possible that you have actual worms. They could be aquatic earthworms, land earthworms (who drown and rot since they're not aquatic), or leeches (ones that feed on debris not parasites). Leeches will stick to the walls of the filter so they should be easy to identify. I just put up my Cyprio filter today. My midge larvae (not bloodworms, another kind) are gone but there were leeches at the bottom of the filter clinging on for dear life! Fish will eat any of the animals I've mentioned which is why they are more numerous in the filter where they are safe. They are harmless.

I have them too Hayley, especially under the gravel in my stream. The ducks love rooting around in my rock to find them (I think that's what they're eating). I appreciate them rooting around because their activity also helps keep the string algae down in my stream.

And the fishies love them too. You can watch them patiently waiting where the stream dumps into the pond. Anything the Ducks miss, the fish are there to get.