I noticed a few months ago that one of my female molly’s right pectoral fin seemed opaque and had some tissue on it, but I wasn’t sure if it was just a genetic/colour variation I had not noticed before. I’ve had her for about a year and got her as an adult.
However, I’ve seen the growth slowly get bigger and spread from the fin to her chest. She seems to have a hard time using that fin compared to the healthy side. She’s otherwise well, active, eating well, has delivered at least one litter of babies since. I’m assuming it’s either a benign growth or slow growing cancer, but I just want to make sure it doesn’t seem like anything contagious. I have multiple other mollies in that tank and none of them have this. I have attached pictures of the growth as well as the healthy-looking left side.