I suppose you could omit a skimmer in a nano or in a large aquarium and some people here do. I think the key lies in the other things done in substitute to having a skimmer for example timely water changes.
Qwuintus,
Get a fan for it. A 3" fan from radio shack is a pretty cheap way to go. And you'll want to keep the MH bulb at least 6" or more above the water level. This should keep your heat down unless you keep your house or apt. warmer than 80 degrees. HTH.