Same thing happened to my fish. I posted some pictures and I'll PM you them. I unfortunately don't have the later stage when her stomach was sunk in. No meds solved it, she just got ill over 3 weeks and showed no bad symptoms other then clamped fins. Then she started to sit at the bottom of the tank more, only came up for food, arched back, and finally a sunken stomach. I removed her and should of ahead of time. Hopefully it doesn't spread but I'll deal with it sooner next time.