NH3 vs NH4+

[Invigor]

what's the difference between NH3 and NH4+ other than one more H atom? Assumingly they're both bad news in a tank, but how is ammonium created vs how ammonia is? how does it convert? in a bag of fish it would be NH3, which is "temporarily" non-toxic, yet adding tank water to the bag would be catastrophic as it would change to NH4+?

can someone help me clarify this confusion?