I used bio-balls in the past, now using bio-bale. Both work fine, but the issue is cleaning. The bio-bale is like a spring-loaded dirt distributer. If you can get your sump out easily for cleaning, it is propably best to do this outside. But my sump is a converted aquarium, so it's too heavy for this. Makes a terrible mess of the cabinet and rest of the house whenever I try to clean this stuff. You don't have this with the bio-balls, it just takes forever to fish them out. So I'm probably going to try the suggestion from Phishless, with the poret foam. It would be interesting to hear if others are using that in their sumps. My only concern would be clogging up if the cleaning is not done religiously. You never get that problem with bio-balls or bio-bale of course!