The tank is just a quarter of a circle, so the volume is 3.14/4*22in*22in*19in*.0043gallons per cubic inch = 31 gallons.
If ammonia is .5ppm, a water change is always in order. When in doubt do a water change. If your bored, do a water change. Water changes are always good
The salt won't help much, and neither will an airstone. The only thing that will help are water changes and time.