Bloodworm?

[BR]

I find these worms every time I clean my first IBC AP filter, I am assuming they are blood worms which means I am fortunate, yes? 

[Chris Carr]

Hard to say from the picture above. Hopefully that is what they are and not a fish parasite like this one : http://community.theaquaponicsource.com/video/camallanus-worms-2012...

[Jon Parr]

Midge Fly larva, IMO. If so, they are harmless, eat gunk, fish eat them up if they can get to them, and really only a minor nuisance as adult flies that resemble Mosquitos that swarm over growbeds and tanks, but do not bite you.