I dont do a substrate, in my opnion you do not need it because you do not need the biological filtration because yo have no load from fish and inverts. and most of the frags are set on eggcrate anyways so they never touch the bottom.
I agree with graphixx22, but however I am using a shallow sandbed to hold the pvc uprights that I put my eggcrate on. I tried just zip tying the crate to the pvc uprights but it kept falling over thus the sand is used.
i actually use a bare bottom in my tank also, although i was thinking of adding a shallow sand bed, just for the look. to keep the egg cratein place, i superglued the pvc legs to it and use a couple pieces of rock to hold it all down. if fact here is the 29gal frag tank ...