Danio with huge bubble on fin.

Hi

I came home from work today to find one of my zebra danios with an enormous bubble on a fin. Water parameters are all fine, 0ammonia, 0 nitrite, 0 nitrate 4 kh.


Regular 25% water changes using ro water treated with marin tropic remineralise.

Apologies for poor photo. Any ideas welcome

could you try to get a clearer photo?

I can’t really tell, the photo is to blurry...