Our 10 gallon came as a "kit" with lights built into the hood and a filter that looks like this one pictured which run across the top under the hood. I never know what to call it. The best I can tell it works the same as a HOB, in that water comes out of the tank, passes through a sponge layer then charcoal then a polishing pad before falling down into the tank. There is very good opportunity for gas exchange as the water is exposed to the air as it runs across the top of the sponges. So what would you call it?