Actually the way it goes is as such
Harmful or not
Ammonium is not (NH4+)
Ammonia (NH3) is harmful
Ammonia/ammonium concentrations are based directly on temperature of the water. The warmer the water the less it ionizes with ammonia as the cation reaction is endothermic. If the water is warmer than the ammonium, then the ammonium will draw energy from the warm water, cooling it, and de-ionize into stable ammonia.
Nitrite (NO2-) is harmful
Nitrate (NO3-) is not harmful
The reason for this is because Nitrite is a highly reactive base where Nitrate is a salt.
Now many people say that Nitrate can be just as harmful, and that's true under certain circumstances. Nitrite is a major component in Nitric acid formation, however, at the correct temperatures (about 70 degrees F to 89 degress F) Nitrate can also become Nitric acid, however, the real danger with Nitrate is the fact it reduces oxygen levels in the aquarium as it displaces O2 in aqueous solution.
Yes, I'm a nerd.