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.
Maybe if he has a minute, @Randy Holmes-Farley could confirm my thinking above, I don't want to tell you the wrong thing, but I'm fairly confident I'm right. I just did a lot of interior painting, and regular wall paint has almost no fumes when applying it, in my experience.
I've just covered the tank with plastic, more to keep paint splatters off of it than anything else.
Just plain old interior latex paint. Now you can choose a low or zero voc paint.
I must have done this 12-15 times over the years.
Open the doors and windows for a few hours.