  I lost a Bekko last week. I gave it a salt dip at 0 % concentration for a couple of minutes. Not quite sure how long, as I was watching the kois and returning them to the pond one by one - but the Aka Bekko didn't show any signs of stress until it was released back to the pond After half an hour or so it was gone. I had tried holding it over the airstone, but the stress was to severe. It was a doitsu Bekko, should have known it wasn't as strong as the other ones. But as you can see of the pictures it didn't have a very good body, but nice spots though The pictures of the gill it seems there's been some parasites activity in them, and maybe it couldn't get enough oxygen  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.  After this a younger koi also got signs of wounds in the tail region, and the fish was covered with a grayish layer (not fungus, more like a tone to the skin). So I figured MG & Formalin wouldn't help it much either, so I gave a 0 % salt dip. It seemed to do the trick. And it also got a dip with the rest of the bunch about a week later, and another one 0 days later (0%). This fish is doing better I guess. Wound seems to be healing. (But the sumi is almost gone (it's 0-0 years), and the sumi on my 0 yo Showas is also faded alot! I guess it's due to the high water temperatures I've had the last week. But the odd thing was that the sumi almost went out like a light bulb on the young koi after the last salt dip. )  So my questions: -Am I right in assuming it was Costia? My first ill koi got sick during a cold water period. The gray layer could also point to Trichodina, but wouldn't all the kois get infected at once with Trichodina? -If it was Costia, am I likely free of it? Or should I do a PP-treatment and kill all good algea, bacteria, and set the whole pond back to start? -Do I need to buy an expensive microscope and learn the business of scraping? (Gilll scraping seems like a hassle...) -Some parasites like Trichodina isn't any problem in small amounts, so as long as water conditions are good, they likely won't cause any issues?  I'm pretty new in the koi buis. Had the pond for a year now. So I'm still eager to learn.  I lost a Bekko last week. I gave it a salt dip at 0 % concentration for a couple of minutes. Not quite sure how long, as I was watching the kois and returning them to the pond one by one - but the Aka Bekko didn't show any signs of stress until it was released back to the pond After half an hour or so it was gone. I had tried holding it over the airstone, but the stress was to severe. It was a doitsu Bekko, should have known it wasn't as strong as the other ones. But as you can see of the pictures it didn't have a very good body, but nice spots though The pictures of the gill it seems there's been some parasites activity in them, and maybe it couldn't get enough oxygen  