Hi, I've used Primafix on three occasions and I'm not sure if it does any good, not for frayed fins/tail rot anyway. It may be Ok for other things.
I used it for a week on my baby moors, well, it seemed to work a bit but one of them didn't like it, swimming with dorsal fin down all week.
I have used it for 3 days in my new tank as one of the new fish had red streaks in it's tail (white tail, very obvious) and a bit of fraying. It has made the red streaks worse and seems to make the tail a pinky colour.
Spotty Junior (calico fantail) also had some fraying in the tail fins so I've used it for 4 days in his little tank. He is the only one in the tank. He is definately not happy and spends a lot of time on the bottom, his tail has developed some red streaks and is definately a pinky colour.
On all these occasions the fish didn't like the medication with signs of fins down. Otherwise, they are eating Ok, etc.
I don't think I will use it anymore for frayed fins or finrot that does not also have fungus. It may work really well on fungus but I haven't had any of that.
ps. I will just treat frayed fins with salt and melafix in the future, it seems to work best. Oh and clean water, very, very clean water.