You're talking air pump for your airstone?
Most pumps don't have a setting on it. If you do have one, like a dial, set it to max and back it off just a bit. With most electric devices that have a potentiometer or adjustment to voltage in some capacity, it's best not to run at max setting. Usually 80-90% is ideal workload, in my experience.
If it's a switch, then I'd go with high or close to high. Hope this helps..