most people buy the ammonia available with no scents or additives. If you shake the bottle and its foamy, not safe to use. Using ammonia is the best way to be able to control the amount added to the tank. It can be purchased usually at walmart, or at places like ace hardware
You can purchase a bottle of ammonia, or you can just use fish food. If you want to go the cheap route, you can just pee in the tank (seriously).