Nitrite should be 0 ammonia 0 nitrate ideally at 20. Temperature can be a factor also, and how many fish you have in your tank.
If your tank is cycled fully before fish are put in, then ammonia and nitrite should not change, remain at 0. Only changes if something has gone wrong, with the water change, or new fish has brought in foreign bacteria or disease, or dead fish rotting.
One thing I notice is, you have quite a few too many fish in a tank of that size. The more fish the more waste produced, if you do not have a sufficient filter and bacteria system you will always have problems with your water.
I also swear by aquarium salt.