Pearlscale Goldfish with translucent bubble around tail

[fleurantia 0]

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! 

[conjmc 0]

I read somewhere that someone popped theirs with a sterilised needle saying it was some bacterial infection on a damaged scale. But I dno, if you google perarlscale bubble in Google images, there is actually pearlscales who have that exact bubble grown on-top on their head. Maybe it is just a deformity?

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[koko 14,341]

@Helen is great with Pearls. I'm going to see if she can help you.

That has got to be the biggest bubble I have seen on a fish. 

[Arctic Mama 3,627]

Oof, yeah. That is a huge one. It could still be snipped and sterilized, certainly, and that is what I would recommend if she doesn’t come back with a better idea. You shouldn’t need to separate the fish out for the time being, though. But we will want to set up a hospital tank with an air stone, heater, and impeccably clean water for afterward. You don’t want to risk further stress to the animal in there. 
 

If you don’t have aquarium salt and methylene blue already, I’d use the next few days to purchase those just in case Helen is busy and we are following my treatment recs instead