I should put ammonium in my tank for better plant growth?
As the rest of the respondents have mentioned, Ammonia should never be dosed to tank stocked with fish.. ever!
The thing is, fish are always excreting urea, which in it's first biological breakdown reaction, changes to ammonia. There is no such thing as 'Ammonium' outside of ammonia in solution with water. It has a lot to do with complex molecular weak bonding, electron transfers, and hydroxyl and oxygen ions. If it interests you, you should take some 2 Year College biology courses, focusing on water biology, if they aren't teaching it in high school. Some Chemistry wouldn't hurt either.
I only mentioned ammonium because in soft, slightly acid water tanks, ammonia ions are present in far greater percentages as ammonium ions, which are not caustic to fish gills, and the plants will prefer to use it over nitrates.
This is not something you should ever experiment with! It's just a natural by-product of the breakdown of the ever present fish, microbiological and plant wastes. In a healthy tank, with good plant growth these levels of Ammonia/Ammonium are at very small levels, you would have to have fairly alkaline tank water and some pretty bad tank husbandry to end up with high, ( i.e. fish damaging..) ammonia ion levels.