For the IQ 30, I removed the bio grid and the media chamber is 2.75” deep and 4.5” wide and about 5” tall. I just bought a semi coarse sponge off Amazon that was 2.75” thick and cut the width and length.
It flows from back to front so sponge and filter floss would work well. Bagged media is probably not ideal. I think sponge is the best media personally because of the mechanical and biological filtration. If I ever needed to run carbon, I’d just pull the sponge and run a big bag of carbon. The filter isn’t really set up to run multiple types of media at once like a AC or Tidal that flow up from the bottom.