  The reason why I gave all my 0 klois a salt dip was that they all were flashing some now and then. I've had them in a 0 degree pond all winter, so I figured it lightly was Costia causing the flashes . I also raised the temperature between 00 - 00 °C (peaked at 00,0 °C) for about 0 days (outside temp. kept me from raising it further), as I've read that 00 °C will kill Costia. But my problem is of course that I can't be 000 % sure it's Costia. It may also be Trichodina, but I figured that Trichodina isn't that a big threat to healthy koi as Costia..?  Water conditions are good. None detectable nitrite and a working bio filtration system, so likely the Ammonia isn't any problem. Also very little foam formation. Water is very soft. I do not use any salt in my pond, don't believe in salting my kois in the long run.  I also lost a fish during early spring. It had been kind of flashing some all summer last year, but I thought some parasites are normal in a bio pond. In early spring it hid a lot under a rock, acting very unusual. Then I realized it hat scraped it's back and gotten fungus infection. Then I realized that my "leave them be, stability will cure them"-attitude wasn't gonna save him, so I took him in and gave him a treatment of Formalin and Malachite Green. But it only survived two-three weeks. No surprise there really. 