I don't think you'll have much of a problem if you're using typical interior latex wall paint, it's pretty safe stuff. But, you can look specifically for a paint with zero or low Volatile Organic Compounds (VOC), these paints don't off gas fumes that may be harmful to the tank. Otherwise, I'd place some kind of drop cloth over the tank, just to keep any drips out of the tank. Maybe turn off the skimmer while painting just in case too, in case there are some fumes in the air in the room.
Good luck!