Most reef tanks either use a
deep sand bed (dsb) or a bare bottom (BB). The deep sand bed is 5+ inches of sugar sand, bare bottom is as the name suggests, no substrate. Most people put down some starboard or cutting board material to prevent scratches. There is an enormous amount of debate about the merits of both ways. The big issue with dsb's is that if they are not properly maintained they become nutrient sinks which eventually release nutrient back into the tank, feeding
nuisance algae and potentially causing problems with sps(hard) corals. BB on the other hand, is in many people's opinions, less aesthetically pleasing, and requires some more maintenance/flow to keep detritus either suspended in the water column, or vacuumed out so that it doesn't break down into excess nutrient.
These issues are further complicated by the fact that different corals have different levels of tolerance for nutrients(softies can tolerate the most, sps the least). Of course If you plan on only having fish at first, you could just go with a shallow bed of sand, keeping in mind that when you switch to a reef tank you'll be removing it.
If you do a search here on "deep sand bed" or "bare bottom", you'll get a lot more info than I can possibly cover here. Overall, I'd suggest taking some time and researching as much as possible before making any purchases...I sure wish I had, could've saved myself lots of time and money in the long run.
