Over the last weekend I have lost 7 koi and am at a loss as to why. A few
days ago the koi in my pond generally stopped eating, having previously been
ravenous feeders. I tested the water and all tests proved "normal" (ph 8.0,
nitrites 0.1, chlorine 0, ammonia 0) except for oxygen which had dropped to
less than 1ppm. I run with 2 filters dropping approximately 1500 gallons
per hour, two spitters and a fountain running at 250 gallons per hour and
can not understand the low reading. I immediately added an air pump with 4
air stones and continue to run that as well, although the oxygen reading has
only gone up to 2ppm today.

The fish that have died have all isolated themselves from the others, and
been very slow moving, although do evade my attempts to net them, right
until they are nearly dead. I have noticed that all the dead fish had a
heavy mucus coating on their skin making them extremely slimy, and that some
of their scales had become very prominent (I have posted some pictures of my
latest loss on http://deadfish.mysite.freeserve.com/).

My pond is approximately 2250 gallons with koi, goldfish and orfe. This
only seems to be affecting the koi though. I am in the UK and temperatures
have been very high recently, with the water temp in the pond reaching about
24 C.

Any help as to how to help my remaining fish, as well as how to improve the
oxygen level in the pond would be greatly appreciated.

TIA