Spanish to English Science Zoology aquaponics

Sus desechos orgánicos, compuestos de ***amonio*** (NH4), son tóxicos para los peces una vez que alcanzan cierta concentración en el agua. Es por ello que éstos se drenan y canalizan hacia biofiltros que contienen microorganismos
llamados bacterias nitrificantes, cuya función natural convierte el ***amonio*** en nitrato de amonio, un “abono mágico” para las plantas.

This is from a text on aquaponics. I'm not a chemist, so I'm a bit confused about the difference between 'ammonium' and 'ammonia' to be honest. From what I've read about aquaponics, the nitrifying bacteria convert the 'toxic ammonia from the fish effluent into nitrites and then nitrates'. All the texts I've seen mention 'ammonia', although they might be using the term wrongly - I don't know! However, the word 'amonio' is normally translated as 'ammonium', isn't it? So, how should it be translated here?

In the first case, 'compuestos de amonio' - I imagine they mean 'ammonium compounds' but it could also be seen as meaning 'made up of ammonium', couldn't it? And in the second, I imagine mean 'ammonia' is converted into ammonium nitrate? But maybe it should also be translated as 'ammonium'! I am very confused!

Many thanks in advance for any help with this! I know I should have studied chemistry at school!!


Sheila