Ewwww. Probably some gammarus. I hate those things. Once you get them in your tank, they multiply like wildfire. I can never get rid of them! I put fish in the tank that will eat them. Woohooo they are gone! Ya right... They have relocated to the filters and deep in the gravel bed. Clean out the filters you say? Changed all the floss, thorough rinsing and inspection of every piece of biomedia (one by one...), and a thorough rinsing of the filters themselves. Even in my bare bottom tanks somehow few survived my "thoroughness" and every time I rinse out the mech pads I find some in there.... Just gave up ... Really sucks when you get it in a planted tank as they love to eat plants (especially any moss you got). Maybe you got lucky and got some seed shrimp? These are harmless and useful as an indicator species and food for fry/small fish.
