I would paint any sides that you won't be able to see through. For example, if the right end will be going against a wall paint the right end as well. It will look great because you will actually be able to see the black side from the front, instead of the mirror effect. Leave any sides you will be able to see through open though, you will find it usefull when trying to look behind rocks and that kind of thing. When you put in the live rock make sure you leave a gap between the rock and glass along the bottom so you can see and keep it open for flow. If you have to lean the rock against the glass don't do it with the bottom rocks, just the ones higher up.