Fungus?

[Ana]

A few years ago I had a fungus problem that spread and killed my entire pond, and since then the new fish have had no disease issues, but recently one of my oldest ones has come down with these marks.
I'm not sure if it's missing scales from chasing the females around, or I got another fungus outbreak in the pond. Any ideas? I'm about to isolate this guy.
He has always been a black goldfish, some gold on the bottom, and and does not look like the others ones who are changing color. The fins almost look like fin rot?
None of the other fish show a sign of the same pattern.
It's a sudden change nothing was wrong a few days ago. I can't take a picture of his other side, where it's more noticeable the white areas.
Yes, he's missing a guill. He's always been like that.

 

[Ana]

It's really hard to get a picture of him.. parents tell me to release him and I will. This the worst part of fish keeping

 

[Mmathis]

Can you give us some information about your pond? How big is it? Type of filtration? Number and type of fish? When you add more fish, do you quarantine them first?

 

[Ana]

Can you give us some information about your pond? How big is it? Type of filtration? Number and type of fish? When you add more fish, do you quarantine them first?

My bad, should've done it before I wrote this in a panic.
So:
-The pond is irregular. I got no idea how to measure the depth of it, I can send a picture for reference though. At the deepest you can fit in it.
-We have a water filter\pump that can withstand 200liters. The water PHD Is at 6.5.
-16 goldfish, 6 river fish.
-Im the owner of the pond, and I am the one who got all the fish, quarantined them when I got new ones, but my brother around a month ago, without my notice or knowledge added 2 (goldfish? Or carp he isn't sure.) without quarantining them. One died, BCS of shock. The other one I've been keeping track and has shown no signs of sickness. Beginning of the year I posted about one I bought more recently, she's doing great
After I wrote this I noticed the little breeding stars they get, so I might just be overreacting. I really fear losing it all because of his mistake

Edit: picture of the size of the pond. I'm