sorry I've never had any luck with fresh water.... my hubby has a salt water fish tank and he has this stuff in there called live rock (sorry he could tell you more than me) and every time you turn around there is a new worm or slug or creepy crawly on that live rock ewwww. I was just gonna say if it is a salt water we get those things all the time too and there supposed to be good to have ANYWAYS...... I will try to make a point...... his brother has a freshwater tank and if no one can give you any advice till this weekend I'll try to remember to ask him about your worm problem?!?!?!