Shubunkin Goldfish Has An Enlarged Backside

Hello.

I couldn't help but notice the weird shape of my Shubunkin goldfish's backside and tail. I told my mother about this and it's been there for quite some time but I had not noticed it before. I'm afraid it might be getting bigger.

Below is the best image I got of the fish as it likes to move around a lot.

Anyone know what might it might be?

To me, it looks swollen but of course it could be just a deformity I've not noticed before. It makes me wonder that if I've only noticed it recently, could it be getting bigger? It's something I need to keep an eye on but I feel really bad for the poor fellow.

The fish is swimming, and eating well. I'm not sure if he's pooping though.

Another one of our goldfish (I don't know if it can be related) is doing long stringy light brown poop. If I recall correctly, the colour of the poop used to be a darker brown and I'm highly certain it's only been stringy recently (past couple of weeks, maybe?).

I'll do some peas later. Can it be any peas or a specific type?

The other possibility is that the fish has eaten some of the gravel. I had a veiltail once, very similar to your fish, who had a habit of eating the gravel - sometimes she eventually managed to choke it up, but, for a while, I was convinced that it had gone further down and she had trouble swaying her tail when swimming and everything seemed to be drooping. I also had a single-tailed common/comet goldfish who had a habit of investing the substrate too. I gather it's quite rare - but it does happen.

Peas are worth trying, but, if that still doesn't work, then live or frozen bloodworm can sometimes rectify the problem and cause a "clearout" of any blockage if there is one. If you happen to eat papaya fruit, then a tiny morsel of that can also work wonders - but it has quite a violent, laxative effect, so I'd only try that as a last resort and be prepared to do a thorough tank clean soon afterwards! 

No need to be nervous about the results - peas are a well-established form of food for goldfish, particularly in this sort of situation, and there won't be a sudden result but hopefully within 24hrs of them eating it.

That's good. If the fish you were hoping would eat it has done so, and therefore any blockage cleared, then it's altogether possible that that's just the shape that the particular goldfish is.