Hi All, \nI have just joined as I read up a lot of information on your site. I have a 6500 litre pond, bottom drain into an Oase filter with koi, goldfish and a sturgeon. I have been devastated with the loss of 8 koi on the last week and still struggling. I have posted on another site for help but we can't seem to work out what the issue is. \nThe koi were found floating on the pond and a goldfish was found lying flat on the bottom of the pond. 3 Koi were still alive and we put these int the QT. I checked water parameters (amonia 0, Nitrites 0, PH 7, KH 300) I have done a number of scrapes, checked the gills and have come up with nothing. The 3 koi did not make it. In desperation I bought the goldfish indoors and increased the water temperature slightly with a little salt and this fish has recovered but still living indoors now in a small QT. My pond is not heated. Another 2 goldfish were also starting to swim on the surface and I popped them into the outdoor QT with salt and they too have recovered and are back in the pond.\nI am struggling with my other koi as another 2 are not doing great and I don't know if they are going to make it either. One is almost floating on the surface and the other looks distressed swimming erratically and coming up for gulps of air. all I can think and all the reading up I have done is this might be something to do with the swim bladder. I have put these 2 into the outdoor QT with salt. My only reason for using salt was a last resource as the pond temperature is only 4 degrees and before Christmas this dropped down to 1 -2 degrees. Te pond does have the air stone on the bottom drain but with a smaller air pump for the winter.\nMy sturgeon has also stopped eating but is still moving around.\nI have tried to give as much detail as possible with the hope that maybe someone may have an idea as to what the issue could be. Any help would be greatly appreciated, Thank you in advance.