In a fuge it's really whatever you want to put in. I'd go with about 4" or so of sand, and a small pile of rock. I really prefer a bunch of 2-3" pieces of very branchy Tonga rock, but now that it's hard to come across, you may just want some small pieces that will be stacked in a somewhat open fashion, to allow some flow through them. If you can get a strong light over that fuge, Mangroves are pretty interesting plants to have. As for algae, don't put anything in there that you wouldn't want ending up in your display, as it WILL make it's way in there. But if you keep tangs, Angels, or other algae devouring fish, you can really put anything you want in there. Chaeto is a very quick growing plant, but needs to be thinned out quite often, as it removes the most nutrients when it's growing. Caulerpa is pretty good at removing nutrients, and also grows somewhat quickly, but has a tendancy to have some periodic die-offs, and can pollute your tank. I know there are other types of Macro algae out there, but they may be hard to come by
