I have a 75 gallon tank with one Oscar, I do about a 75% water change once a week and filter tear down once a month. Last week when I cleaned one of the filters on the water polishing pad were a couple small black worms. About the size of blood worms but black and moving. I've never seen this before and wondering what they are, if they can hurt Oliver, and how I can get rid of them. Thanks for any advice!

a pic would be good but usualy they are tubifex worms/freshwater aquatic worms..commonly known as "black worm" they usualy live in dirty water courses such as drains etc ..so a dirty filter wool would fit their teritory perfectly .. - or they could be some type of midge larvae. Most of these are used as fish food as well.