Airstones come in a range of sizes but if you only have a small tank, you only need a small airstone.
If there is too much water movement caused by the airstone, the fish will be constantly swimming into currents and that eventually stresses them and can kill them. The fish need calm areas where they can sit and not be washed around the tank. If you don't have any calm areas, then get a smaller airstone or turn the pump down.
The fish seem to love playing through the bubbles and it goes straight up the back so I don't think it causes much current. I was more concerned about the sound of it irritating them at night. Is it ok to stop it at night and turn on again in morning?
Deanasue is right. It depends on the fish. Look into the natural habitat of your particular fish. Swordtails love currents and lots of air as they come from streams in the wild. Bettas not so much. They thrive in a still tank. Fish like goldfish that gobble up oxygen and excrete a lot of waste benefit from the added oxygenation and the surface agitation that gasses off some of the waste. It's so rewarding to learn about your fish and what things you can do to make them thrive.