I have to second the idea that fish nipping often happens when nobody is looking. Since you still have 2 together, it is very likely that those 2 are figuring things out between themselves. With angels, it is not uncommon to see random fin tears now and again. If it was fin rot, you should see an edge and a distinct "melted" appearance one the fin.
So, a few things. First of all, .25 ammonia is in no way an issue. Just so you know. Don't fret about that. It does sound like your tank is not cycled though, since you have ammonia but not nitrates. So, I suggest looking into cycling. Second, the actual healing properties of stress coat are debatable if they even exist. That stuff is first and foremost a water conditioner. Once your bottle is empty, I would suggest something like Prime that is more concentrated and actually a better deal in the long run. Fish have immune systems that will fight off small infections as long as the fish is healthy, and the water is kept clean. I would just focus on keeping your water nice, especially if you have to cycle the tank still. Yes, it is indeed totally normal for a fin tear to heal quickly like you described. It is actually a very good sign for your angel's overall health.
Does your tank have enough space and cover for the two fish together to escape each other if necessary?
