White splotches on black fishes

[NewFishBoy]

Hello!

I wanna prephrase this by saying that we're complete beginners - we moved into a new house a little under a month ago, and the previous owner gifted us his Kois and other fish, that live in the included pond.
We love animals, but the previous owner wasn't much help in regards to care and we devoured as much information as we could as quickly as we could. The water looked terrible when we got here. We managed to get it clear within 2 days, but haven't gotten to reading the exact water quality yet or take measurements as we're still kind of in the middle of moving everything.
However, I'm afraid the fishes have a sickness that needs quick help, so this is why I'm posting already. Hope this is alright.

Information I do have (bear with me ):

• The previous owner could only for sure tell us 5 of the fish's species: 5 Kois, 2 that look similar but are smaller and differently shaped, several (at least 15) smaller black fish. (Pictures below of most of the fishes)
• We haven't used any water additives or conditioners yet.
• The owner left us one bag of a german product from "Fit + Fun", it's a pond stick mix, and we feed them once a day about 2 plastic cups.
• No new fishes have been added.
• No medication added to the pond.

Information I can add later today:

• Name and size of the filter(s)
• How often the water gets changed and when the last time was.

As far as I can tell, this has only affected the black fishes at the moment.
A few one of them have small white splotches on fins and body, one of them is especially affected and I was quite shocked when I spotted it.
Here are two pictures of the sick one:

I looked online, but couldn't find something that I felt looked like this 1:1, and I just want to make sure to figure out precisely what these little guys have so I can get them exactly what they need.

And here some of the other fish (of course the lighting had to be terrible now, but I hope it's better than nothing):

The water is much clearer than these pictures make it seem, I promise.

In any case, thanks to anyone who read this far, despite how little information I can give at this time. It means a lot!

 

[AlyssaFish]

I've never seen a disease like that. It might be some sort of body fungus and it just looks strange in the photos, but I don't know. It could also be ulcers or blisters that, again, look strange. I can't even tell what species we're dealing with here! Any chance you could get better pictures?

 

[NewFishBoy]

@AlyssaFish

Thank you for your reply!
I will be able to take better pictures (hopefully) tomorrow, when I will attempt to catch the really affected fish, to separate it from the others.
I would've tried to already, but the previous owner only had a tiny, flimsy-looking net. The fishes were laughing at me lol.

 

[Mmathis]

Hello and welcome!

I am not at all good when it comes to fish illnesses. It’s kind of hard to tell anything from your pictures, though. Would it be possible for you to catch one of them, for a better look? How are they acting — any unusual behavior?

Do you know the size of the pond?

 

[j.w]

@NewFishBoy

 

[NewFishBoy]

Mmathis
Thanks for your reply!
Unfortunately, I didn't get to catch them today as I had hoped, but their behavior is not unusual from what I can tell. They eat and swim around energetically, they don't avoid each other either.
The pond size is an estimate, but our guesses are between 60-80m²

j.w
Thank you!

 

[NewFishBoy]

AlyssaFish
Mmathis

Hope it's okay I tag you both again.
I still didn't manage to catch any of them, but will hopefully get some better equipment tomorrow.

In any case, I did manage to take some better pictures of the heavily infected fish with the sun out today:

I hope these work a little better until I finally manage to catch this slippery friend.

 

[IPA]

Have you tested the water? What are the results. I don’t keep koi but are you positive the fish just is not changing color? The white is obvious but any sign of illness or distress isn’t.

 

[Mmathis]

@NewFishBoy I’m sorry, but I am terrible at math! You said the volume is estimated between 60-80 square meters, so I’m assuming those are the dimensions since meters are length. How deep is your pond?

 

[AlyssaFish]

I don't know anything about Koi (which is what that appears to be) and this doesn't look like any goldfish disease I'm familiar with. I would have thought it was just the normal coloration except there's something about the lumpiness around the tail area patches that bothers me.

The best treatment for any disease is good water quality. Barring that, isolating the fish individually for treatment in quarantine tanks is a good idea. There are anti-fungal medications you can use, and you can use medicinal levels of aquarium/pond salt.

You're sure it didn't just roll over in some powdered sugar?