FroggyFox summed it up pretty well. Here is something to consider. An aquarium has several different areas that fish prefer, bottom, mid level, top, glass. If you have fish in all of these zones, then you have a very full active tank. Scavengers generally tend to prefer the bottom, so if you want fish in the bottom of the tank, why not go for some scavenger type fish. I always specifically feed my scavengers though (sinking pellets, bloodworms, etc), so it's not like they really have to fight for tyhe food that comes down to them.