Fish does not look good

[Ronfire]

I have goldfish and Koi in a pond that has been running for 1 year now. The goldfish I have had for 3 years in a smaller pond that is now rebuilt to around 3000 gallons between 2 ponds. Water quality is ok, low nitrates,low ammonia,low phosphate, ph has jumped around between 6.5 to 8.5 Filter is a bog equal to the size of the 2 ponds as well as a sand and gravel filter I have been having a lot of string algea All the fish are moving and feeding well but I have 1 Koi that looks to have mouth fungus and I saw this goldfish with patches on his side . Not sure what to do with the pond for treatment, Catching the goldfish is impossible as they are fast in the 5.5 ft deep pond. Here is a couple of pictures with my Gopro in the pond, not the best to see the issue.

 

[IPA]

I’d guess the best treatment would be to get the water quality as perfect as possible and hope the fish’s own immune system can fight off the fungus. Add baking soda to buffer the pH? Has the water been warmer than normal? Add an air stone air diffuser? Do you think it could have been caused by injury, maybe a predator?
Ammonia and Nitrite need to be 0 , if low is 0 then ok.

 

[Ronfire]

Nitrate and ammonia have always been 0 Phosphate 0 130dkh .The water has been getting a slight tannin colour, I think from the dying algae. I have been doing a steady slow water exchange for a few days from the well and the water is nice and clear. If I could catch the fish I would try a salt bath. The water temp is not to warm , the outside temp have just started to to 75f.

 

[IPA]

Do you have an aerator or way to increase dissolved oxygen? Why is the algae dying, did you use something to treat it?

 

[Ronfire]

I did not place any treatment in the pond, The algae is still growing but there is some brown algae as well, maybe it just makes the pond look brown now that the walls are not so green. I do not have any air into the pond but the water is very aerated with the 5 water falls. When the water is calm you can see the surface has tiny air bubbles coming up from the waterfall, like the top of a soda glass.There is a current in the pond when you look at the string algae so there is good exchange and have a 3500gpm water flow.

 

[Tula]

As @IPA mentioned you need to buffer the pond's water. What is the KH, if you keep it high it'll prevent PH swings, which are harmful to the fish.

To my eye this appears to be fungus . If these were my fish that's what I'd research and treat for.