Becky
You definitely want to keep the dust out of the tank. When it contacts the water, it will become drywall mudd again. Drywall dust is so fine, it will travel everywhere in your house. I would shutdown your heater/blower while sanding. In large computer rooms, we only allow wet sponge sanding if work is being done around our equipment because the dust can be sucked into the computers and settle on things. It is not really corrosive, but will reduce the cooling by covering all the components.
Someone recently posted a complete tank crash from work being done on his house. If I remember, it had to do with staining wood, but I would be very careful letter any airborne construction debris from getting into your tank.
You can reduce the dust a lot by getting a little thing they sell at Home Depot / Lowes in the drywall department. Its a sanding pad with a hose that draws the dust through a bucket of water and traps it. It is powered by any shop vac.
Another way to keep the dust down to a minimum is to clean as you go. Slowly sweep up the dust and get it out. Walking around on a floor covered with drywall dust will just kick it up into the air more and more.
If it was me, I would seal the fish area off with plastic sheets hanging from the ceiling and also cover your tank and sump directly.