You do need bottom-eating fish for your aquarium. The fish that are associated with eating on the bottom are typically catfish. A lot of the catfish get very large, so keep that in mind when you're buying bottom-eating fish. Ask your sales rep at the fish store, "How big do these fish get?" A lot of the Corydoras Catfish are preferable because they stay small, they're very peaceful, they have very small mouths so they'll just spend all day long trying to find food in the gravel.
Remember, the food is only good for maybe an hour or so after you feed it, otherwise, it just turns to mush. The detritus, the fish feces, all that stuff is going to settle on the bottom. Having these catfish constantly probe the bottom will keep the bottom looking clean and keep a lot of that waste in suspension where the filtration can get to it. When picking bottom fish, you want to use the same guidelines that you would when picking mid-water fish: Fish that don't get too large, fish with small mouths so they don't eat your fish.
You should mix up the variety, too. Maybe some algae eating fish, like plecos, or algae eaters. Remember the plecostomus, or plecos, get really big, so keep that in mind if you have a very small aquarium. Again, Corydoras Catfish is what I would prefer. Just a small school and they look really cute.