fleurantia

I have 2 Pearlscale goldfishes and I noticed today that one of them (age 4) has a big round red translucent bubble around its tail. I'm really worried that she's sick. Can anyone tell me what this might be and how we can treat her? Do we need to keep her separate with our other Pearscale Goldfish that's in the same tank? Thanks in advance for your help!