I use perlite, you can get a huge bag of it ordered through Home Depot pretty cheap, if you look it up at HD online it comes in a generic looking green bag...the one I ordered turned out to be about the size of a 30gal kitchen trash can. Had I known that when ordering, I wouldn't have ordered two...lol. I put about two inches of perlite in my egg boxes, fill water about half way up the perlite, one layer of light grate on top of the perlite.