I think that if you have some agitation on the surface of the water you should be good. Running a tank topless with a powerhead creating some surface agitation is fine for most people. There are pH buffer though that may help if your pH is off... be careful with that stuff though. Remember too that pH can change with temp and time of day, and even things like having a window open nearby can have a big influence on pH as well. I think as long as it is stable at 7.8-8.2 you should be fine unless you are keeping finicky SPS and other stuff like that.
Jesse
