The only good reason to do terracing if you're keeping shellies is to plant behind the terrace. The Shellies will dig anywhere there are shells, and that means that if you put shels on the high side of the terrace, they will just dig on both sides and spit sand back and forth over the top. If you want verticality, go with a rockpile (or a stand of vals if you think you have the lighting.) Alternately, some well cured driftwood would not be totally out of line so long as you use calciferous rocks (tufa, lace rock) to counteract the acidifying properties of the wood.
You know, you could put a catfish in there and that would keep your fry from getting totally out of hand. A secondary rockpile concealing a half flowerpot would work nicely to provide a hang out for it.