The nitrate sponge are little beads of nitrate removing media. You can place it in a mesh bag and add it to your sump. or run it in a hang on back filter as well.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.