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.