Personally, I have never felt the need to run an air pump at night. Turning off your CO2 1/2-1 hour before lights out pretty much uses it all up before the plants' aerobic cycle. A little surface agitation can't hurt either. I've always felt that good turn over between the top and bottom of the water column is important. I have not tried to verify this in anyway, so I can't really claim it has any great benefit. Water at the top of the tank is always going to be in better equilibrium with the atmosphere at night however.