You'll know when you have ammonia in the tank. Smell the water. If it has an oder , be quick to do a water change. You don't always need test. Good to do. But if you don't have any. Just watch your fish, smell the tank anything different change the water. The ammonia usually will come when over feeding, or something added to the tank. It will kill your fish. The nitrate can not be high. Keep it low. the nitrite is like your ammonia just not as toxic.