Agree with what has already been said - airstone depends on the size of the tank and how much flow - if you have lots, you'll probably be OK, but in a smaller tank like a nano, an airstone is a good safety precaution.
I always recommend dosing the stuff in the morning, after the lights have been on a while. It does drop the pH (or has the potential to) a bit, and pH tends to drop at night anyway, so doing it during the daylight hours, mitigates the risk of a pH swing.
Jenn