That's a false wall to the in tank filter. Yeah, it sounds like you have some live rock rubble, a sponge, and bio balls.
In saltwater there are many different filters. Sumps, are separate tanks that run tuns of things. HOB
filters like AC
, commonly used on NANO tanks. Intank AIO filters, like you have, where everything is in the tank. If I were you id get rid of the Bio Balls and the sponge. I've never run an AIO, but Carey, has run bio cubes and can probably help here.