Hi - I have a 2000 gallon pond with 5 adult koi and about 5 fry. The pond is in the shade and does not get direct sunlight. It has a waterfall and bog, and a large aerator with a Pondmaster Clearguard pressurized pump. Fish are active and eating normally - KH is about 6 drops and pH is 7.6, high range pH is 7.8/8, ammonia is 0, nitrite is 0 and nitrate is 0-5. All but one of the fish were spawned in the pond and the KH has never bothered them.
I changed about 20% of the water last weekend and a few days later thought my large white koi had a slightly pink dorsal fin but thought I could have been imagining it. Today it seems more pink and I think it could be a parasite problem but don't know. I don't have a microscope and have never examined them out of the water. They have been healthy with no problems over the last 8 years and I have never added anything to the water including salt or baking soda.
I've attached a photo of the fish in question and am seeking advice about what to do - wondering if I should try adding salt. I don't have a quarantine tank but do have a bubbler and large rubbermaid tote in case quarantine is suggested, but think perhaps I should treat the whole pond.
Thanks for the help.