1. put bioball inside ..
2. put 2 layer of cotton wool on top of the bioball to reduce the waste + more even water going down. Change the wool every 2 weeks. The wet cotton wool also help to reduce the water falling noise.
3. Install a ball valve in the sump , close a little bit so to slow down the water from overflow -> sump.
My tank used to setup as the above method until I install a durso pipe ...