Fin Damage

Stress coat does claim to 'reduce stress by 40%'. The website goes on to say that aloe vera does it by forming a synthetic slime coat. Prime also 'promotes the production and regeneration of the natural slime coat' so if that's what you have I'd carry on using it. Personally, I use a dechlorinator that does not contain aloe vera as I don't want it in my tanks.

As for the fins, yes that is typical of fish that chase and nip. A lot of shoaling fish use this as a way to set up and maintain a hierarchy within the shoal. It could be that the water changes are stimulating them in some way, and that is causing the behaviour to be more pronounced. For instance, cory breeders often do large water changes with cold water to get their cories to breed - the cold water simulates the melt water coming of the snowy mountains in spring.
Water changes are actually the best way to deal with this. As long as the fins don't show signs of infection, keeping the water clean is the best remedy to stop an infection getting in. Things to look for are white, black or red edges to the fins, or fins that are getting visibly smaller. And because it is only the tail fins, that does suggest nipping. With finrot, I would expect more of the fins to show some signs.