if you put the outlet under the water be sure you have an air stone in your tank.

You need to be sure you have some form a surface agitation as this helps to break down and disperse harmful gasses that build up in the water (C02).

You dont need to have it running all of the time, you could turn it off (air pump i mean) at night while you sleep then run it during the day.

Be aware that the smaller the tank the less water you have so the easier it is for bad chemicals and gases to build up.