Had just set the rocks in like that. Made sure everything was connecting on at least 3 points so that it was stable.
I ended up taking the rocks out though, too much floor space covered and the sponge filter didn't provide enough water movement to flush detritus and food out of the rocks. Was a pain keeping it clean.
I think I'll be upgrading them to a 40 Breeder in the next 4-6 weeks though, which will give enough water volume to have a decent canister filter on the tank (more water flow). Will set up the rock caves again in that, though I'll likely user larger pieces this time. I got a rock-cutting blade for my sawz-all so I can make a bunch of 6"x12"x.5" shelves and stack them. Will dremel them out a bit too so they aren't 'machined' looking. I can always use the canister output to flush out the rocks every few days too and keep stuff from building up in them. Think that will work out pretty well.
For now it's just black sand and some black PVC caves. I did upgrade the filter to a Fluval U2, and added a 27" bubble wand burried under the sand. Bubbles coming up out of the sand looks really cool. More water movement with those, but I still don't think it would be enough to keep the rocks cleaned.