yea the crushed coral is ok, it'll hold detritus a little more because of its shape, but also becaus of its shape it'll give little cavities for critters to hide in. and yep southdown for the fuge is good, put that in to a 4-6" depth and a couple of your smaller rocks, or your rubble from the rocks you get, and some macro, chaetomorpha works best because it wont try to sexually reproduce killing itself and making your tank dirty. as for online rock, mine came from www.liveaquaria.com and im pleased with it and i think most others have been too.