I just recently purchased a pregnant Gold Dust Molly. I also had put in a heater a week before I bought her, as the weather here is getting colder during the night. My veteran Black molly loves to hang out around the heater, but she's never gotten burned. A couple of days ago I noticed my G.D. was swimming around with one fin stuck to her side. I had previously read that this sometimes happens, so i kept my eye on her. I've seen her swim near the heater, but never for a long period of time (unless she got burned while I wasn't looking, like what happened with her birth.)
It's been about 2 or 3 days since I first noticed her "stuck fin", and now she sits on the bottom of the tank by the plants, not swimming. Her fin now barely is even there anymore, it's all white and it looks hard and crusty, like a burn would. It's definitely not fin rot, no other fish is experiencing this. There's also a white crusty spot that showed up on her top lip, next to what looks like a bloody nip.
Tank mates include 3 other mollies (all female), 2 platties, a guppy (all male), 2 rainbow sharks (but they have their hiding places, they dont bother any of the fish) and pictus catfish (which are getting rehomed today to prevent overcrowding). 30 gallons, parameters all good, just did a complete gravel rinse and rescape/tank drain 1 week before I bought her. No other fish is affected with this weird behavior. Did she get burned or is she being bullied without me noticing? And how do I treat it? I appreciate any info, and if there's any info I need to add, let me know.