Fin and tail rot, or ammonia burn. please help. need to know soon!?

I have a comet goldfish, shes in the tank with another goldfish, a cory catfish, and a small pleco. I first started to notice little white dots on the ends of her right fin, but just on the ends. I didnt know what it was so i kept an eye on her, I woke up yesterday morning and her fins were torn up, the base of her tail was bright red, she had red streaks almost, kinda looks like blood, going down her fin. as well as an open red sore on her side. I immediately checked the levels in her tank, her ph was off so i did a water change, hoping it would help. I redid the test and checked the ph and the ammonia, and everything was fine, but now i see my other goldfish is starting to look like she did when it was first happening to her. I did my research, lots of it, and discovered that ammonia burn can cause many symptoms including red streaks in the tail and ragged fins. But I also read, in many locations, that fin and tail rot can be a result of ammonia burn because the fish is left weakened from the ammonia, or something like that, is this true? I'm very confussed, because if it was ammonia burn the streaks and everything should calm down once the ammonia levels are back to what their supposed to be, which her streaks did calm down, but now my other fish is getting ragged fins too. Is this ammonia burn or fin and tail rot? what should i treat her with? i have api tc (have no treated yet, but i read that api em works better. what do i do? please help. (sorry its so long, i wanted to give as much detail as possible so i can narrow it down)