Bare bottom is best. If you can't stand the look, then a thin layer of sand (not 2"!) is fine. Heavy layers of anything will trap debris. Gravel is by far the worst substrate anyone can use. It will trap everything in sight, and is almost impossible to thoroughly clean. I've seen people in the past complain that no matter what they do to their gravel bottomed tank, they cannot lower nitrates. I told them repeatedly to lose the gravel, and they swore they kept it clean, and that it was not the problem. When they finally did remove it, they could not believe how bad it smelled, or how much crap was trapped underneath that they couldn't clean before. "Magically", after removing the gravel, their nitrate problem went away.