a small separate tank, or section of your sump, containing sand/liverock/ and a macro algae, ideally cheatomorpha. It siphons out what makes algae grow [phosphates] and cleans your water of those elements so algae doesn't grow in your display tank; it also grows/breeds small crustaceans called copepods that get through your return pump, enter your main tank, and feed your fish, particularly fish like mandarins and other dragonets.