Jayde

Hey there, Long time no visit, so long in fact I'd forgotten my login details. ? Have been out of the fish keeping for about five years, but just got back into it last year. I have a neon rainbowfish that I've just noticed has an odd discolouration at the base of its tail, its on both sides. To me it sort of looks like a dark pimple? I'm guessing maybe a parasite or worm? Any ideas? I've attached the best photo I could get. Cheers Anna