Pot scrubbers are good for coarse filtering/pre filtering. I bought 2-3 packs of these:
Foam Blocks, cut out of these sheets:
You got a great idea with Lava Rock and Ceramic Rock, great for Beneficial Bacterial Growth.
My sump is 2 x 20 Gallon Tanks, I run, from beginning to end, Top to bottom:
First Chamber: Pot Scrubbers, Foam Block
Second Chamber: Lava Rock, Foam Block
Third and Fourth Chamber: Refugium, Approx 10 Gallons Total, My heater and an Air Stone are in here.
Fifth Chamber: Foam Block, Ceramic Noodles, Activated Charcoal
Final Chamber: Foam Block, 2 x Bonded Filter Pads