White marks on orando fin.

[Kenton12]

I'm reasonably new to keeping goldfish so it might be obvious to everyone else what's wrong with my fish. Apologies for the low quality of the photo but he just won't keep still

The ammonia levels are fine as are all the other fish in a well aerated 50L tank. One of my other fish had fin rot last week but I treated it immediately and it has now recovered. I only noticed these white marks on the fin of the fish tonight and have been searching online for pictures that look similar, but I'm not able to diagnose what it is.
The marks look to be raised slightly and white when reflected in the light.
I hope the picture is clear enough for someone to point me in the right direction as to treatment and the cause of the marks.

 

[jlk]

Hi! Welcome to AA!! Can I ask exactly how many fish are in this tank? What are your exact numbers for ammonia, nitrite, nitrate & ph? What are you using to test (liquid or strips)? How long has this tank been setup? What is your water change schedule (%/#wk)?

50l is only a 13g tank and I am suspecting that your water may be the primary source of your issues here especially because you mentioned another fish with fin rot. Some more info will help us to help you better!