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LookinForNemo: If your tank has only LR
and substrate, i would swap your substrate now. Swapping the substrate can cause your tank to go through another cycle. If done properly that can be minimized, but it is still likely. Because of that you would be much better off swaping the substrate before you add any other livestock to the tank.
Ixmnrs: the other advice so far is spot on. As far as gobies and sand-sfiting stars... you might want to re-think them. The gobies and Sand sifters are actualy eating the life forms in your sand bed when they're "sifting" the sand. The life forms in the sand bed are what make the sand bed work. If you like those animals you'll need to be able to supliment the life in the sandbed by replacing their number through a refugium etc. you can read a good amount of that info on here by searching in the salt water forums (search link is at the top)