Crimped Fins on a Common Goldfish?

[Aimee]

Hi!

So, I've taken in a coworker's goldfish who was getting too big for his tank. I built a 150 gallon "pond" for my current common goldfish, Obie, who is about 7 inches long, so I figured I could try homing them together. Right now, the new fish, Jonathan, is in a 20 gallon quarantine tub with an established sponge filter. He's been in there a week, no ammonia, no nitrites, low nitrates. I went ahead and gave him a part of the quarantine trio to make sure he's all good. Behavior-wise, he's a little shy but he's gotten more outgoing with time.

So, my question is: Does anyone know what's wrong with his fins? He was given to me with these weirdly crimped fins and tail, like it's been folded at some point. I've looked around online and I can't find anything like this.

Here's a video I made of him as well: 

Thanks for your help!

Edited March 6, 2021 by Aimee

[Colu]

It could have been stunted by being keeped in to small a tank for to long time

[Aimee]

That's what I was thinking too. Maybe a deficiency in nutrients or something. She acted like his tank wasn't that small, but it does make me wonder...