sand or bare glass bottom?

[Whatigot]

Sand can actually help a lot.
Aragonite breaks down, introducing trace elements and calcium naturally to your system.

It also can host a number of living organisms that are beneficial to your system as a potential food source for inhabitants or as detritus/waste eaters.

If you have enough sand, it can act as a significant addition to your biofilter as well and help give beneficial bacteria surface area to populate.

I will always put sand into any reef tank I have, not just based on how much better it looks either.