What exactly does this object do?

I think I may have made a mistake in not using one. I figured the bio filter was the flower bed that contains my hydroton. But I'm starting to think now it's some other object required in an aquaponic system?

I do have a Hang-on-back filter. But the filter I have in it is just a sponge thing that catches big solid waste particles.

Can someone give me some advice on the matters place?

Thanks for reading!