sand or bare glass bottom?

[2manytanks]

I also am partial to sand. As someone else said, if done properly it also adds to your capacity of biological filtration. But do a bunch of reading on the internet about sand beds and make up your own mind.

You'll hear a lot of people talking about the lower levels of sand going anaerobic but with the depth you'd likely be using in a nano tank that shouldn't be a problem. In any case, with a deep sand bed the lower anaerobic layer is important to it's proper biological function. Once established, the bacteria that populate the top will remove ammonia and feed nitrite to the next layer down which is populated by bacteria which convert the nitrite to nitrate which is consumed by the anaerobic layer. But to work properly the sand not only has to be live with bacteria but other critters like worms which will channel and mix the sand to allow flow between the different layers.

It also opens up the possibility of having other animals like sand sifting stars, gobies and sand dollars - who will also help keep the sand mixed and clean. With a shallow bed in a nano tank you'll want to keep the sand stirred up to prevent the build up of detritus and stars and gobies can fit right in. I had a 12g tank with both a sand sifting star and a goby, they did just fine and it was a beautiful tank.