External parasites on FW fish?

[fishoholic]

To me it looks like a scrape/cut of some sort. If this is the case the melafix will help heal the tail in about a week. It sort of looks like she got her tail caught or stuck on something and ended up hurting herself pulling free. I can't say for sure but I hope the melafix works for you, it has helped my FW fish in the past.