almost anything cango in a sump to be exact.. there are plenty of di. types of bio media.. there are bio-ball,bio-cubes,and whole buch more.. even thingie that look like plastic stripes or shavings. anyhow.. inmy sump i have a skimmer and a metal heater w/bio-balls.. it's like the same thing as putting them in the display tank..just that it's in a hidden place leaving the display tank looking neat w/no equipment..