No phosphate in ammonia/ammonium.
Ammonia and ammonium do not act the same to harm fish and their gills.
This is going to be simple, but micro algae (zoox/dinos) cannot get nitrogen from nitrate. Some coral hosts can convert no3 back to ammonium, but it costs them a lot of energy - some cannot. Assume that dosing no3 on the back end does not help the zoox - this is not completely true in all cases, but it won't hurt to do this. Micro algae needs to get their nitrogen from ammonia/ammonium.
You need some no3, but only to build/repair tissue. It is not food. It is not fuel. A trace is enough for this. Excess can do things like poison dinos and some matting cyano bacteria, but it also can inhibit calcification and interrupt biological processes when it gets too high - the level of badness is different with each organism. Macro algae will love your for increased levels of no3. Most of the time, I guess that excess no3 just builds a larger anoxic bacteria population which is why so many people have had to add more, and more and more as they dose it.
I understand that this might be hard to get a grasp on. I think that the recent trend to raise the N and P in a tank is because people can test and put a number to something, but this is somewhat of a simple exercise... the prize is in high availability of ammonia/ammonium for coral use and then low residual levels of N and P so that calcification and cellular activity is not slowed down. Heavy import and heavy export. Of course, people cannot measure high import and high export, so it is not as easy to type and parrot on a message board. The people who dose N to the back end are not likely doing what they think that they are doing. The people who have raised no3 by feeding more have certainly had more availability for their corals, which is good and the availability has helped them, not the residual buildup on the back end.
I dose Ace Hardware ammonium into my frag tank when I get new clams... seems to help then get started. This is all anecdotes and nothing scientific. I add 2 mls of the stuff (10%, I think) into a 60g frag tank a few times a day when I remember (300g system). I can never detect any of it.
Remember that some single cell stuff, bacteria and some types of algae all will really love the ammonium too.
I have always wondered if I can dose ammonium in a ATO with RO, or if the ammonium will break down over the week. This would be the easiest for me if it would work.
FWIW - freshly mixed Instant Ocean has some ammonia in it. I always considered this a good thing.
In the end, the people who have always told you to feed more and skim heavy (or otherwise clean up the back end) were always right. This is why I think that dosing N and P is just a fad that will stop one day.