July 26, I added three newly purchased 1” koi to a 75 gallon tank that has four larger 4” koi (3 black Karasugoi & a Doitsu Taisho(mostly scaleless)). Yes, in hindsight I should have used a quarantine tank.
30% water change was done on the 27th using API Stresscoat and morning of the 28th two of the small koi had a handful of tiny white spots on their fins. I dosed the tank with non-iodized salt (tablespoon per 5 gal as a start)
Water temperature was about 78°F
Spots were all gone the next morning July 29 (seems too fast for ich to disappear). Then July 31 the Doitsu Taisho was covered with dozens of tiny white spots. None of the others had any spots although the Karasugoi appeared to have a thicker than usual slime coat.
They all had voracious appetites, no clamped fins, no flashing, no lethargy.
Gradually increased the salinity to 0.30% and the spots dissappeared for the most part, except for a very few on the tail of the Doitsu Taisho. It has been moved to a 20 gallon quarantine tank for more than a month now with salt increased to 0.50%.
Spots only seem to be on the tail, and seem to be located in only 2 areas – near the top and near the bottom, both about half way along the tail. Spots grow over a few days then disappear, but there always seem to be a few in one area or the other or both.
Water temperature now, (September 23) is down around 64°F and the individual spots seem to grow slower and last longer (about a week) before fading
Other than these spots the fish seems fine. No signs of stress, active, eating well.
This is the first koi problem I have had in over 20 years - I'd really appreciate any comments, suggestions, or a diagnosis if possible