Ammonia!!!!!!!!!!! how stuff works.

[Sk8r]

WHY people fail at hobbies and blame themselves...
It's the things they carefully don't explain in beginner instructions.

Ammonia. Lethal to fish. They give it off, don't thrive on breathing it. Die of it.
Ammonia is (formula) NH3. (N = nitrogen, H = hydrogen), and is lethal. NH4 (ammonium) is not-lethal.
NH3 kills. When you pour in an ammonia remover like Prime, it says it renders ammonia (and nitrate and nitrite) harmless. It converts NH3 to NH4 BUT!!!! that conversion is not permanent. It does that for 36 hours.
So----novice fishkeeper thinks he's solved the problem.
Nope.
You have a cycled tank. You have companies and battalions of bacteria trying to suck up that bad stuff. They eat ammonia. But a newly-cycled tank may not have enough battalions yet.
Where a water conditioner (Prime, Ammolock, etc) works is this: it converts that ammoni-stuff into stuff bacteria can eat: if you have enough bacteria, ie, if your tank is old and strong enough, your rock bacteria can take care of it, and it will be fine.

Also---all the ammonia monitoring strips and badges don't tell you---they can't tell the difference between ammonia and ammonium. That's not real helpful except in one thing---if you have a toxic level of ammo-something in your tank, you have a potential for real trouble---if your rock bacteria aren't enough yet to eat it all. And you might think that activated carbon, touted as something that removes chemical problems from your tank, will remove ammonia and nitrate and nitrate. Nope. It doesn't.
So----what they SHOULD tell you is that a) there IS a more specific test (which you can buy) that can tell the difference between ammonia and ammonium (ammonium can come in as plant fertilizer.) That's of use to know.

And if your tank bacteria are strong enough and numerous enough, they can scarf up that converted stuff and get you out of trouble before the conversion-magic expires and it becomes nasty again.

But if the problem is too much for your new tank?
The way to solve the problem altogether in a critical situation is water change. Just plain suck a significant amount of water out and put clean, ammonia-free, dechlorinated (ro/di) water in.
A maxim for all tank lovers is a little rhyme: Dilution is the solution for pollution. And that covers all sorts of situations. Dilute the bad stuff.

A few takeaways:
1 fish give off ammonia and they must not breathe it.
2 Shipped fish in bags accumulate ammonia as ammonium, and are fine with that. BUT when you open that bag that ammonium starts converting to ammonia real fast. You have 30 minutes to get that fish into the tank, and drip acclimation that takes longer than that may actually kill the fish. [I always hate the stories that begin 'but I acclimated that fish for two hours...]
3. Prime or Ammolock or the like is a must to have on hand: it can buy you time to take action to SOLVE a tank problem with water changes.
4. Water changes dilute the problem down.