+1...another option which i use in all of my ACs - Blue pad on bottom, sponge, some poly or felt pad and then a bag of matrix. Besides their overall performance, with the AC filters you can add just about any type of media to fit your needs...
I've had great results with using the foam block then filling the rest of the space with a net bag of Eheim SubstratPro, used two like that on a very heavly stocked 45-gallon about a year ago and it worked great.