New pond problems.

Firstly I would like to congratulate you on such a wonderful web site, it has been a great source of both information and inspiration to me in building my first ever pond, well done! Secondly I would very much appreciate some advice on the following:
I completed my pond in August '99. It is 6 feet deep, 1850 gallons, 1 bottom drain with 110 mm pipework to a vortex, a brush chamber and 2 chambers of japanese matting.
The last chamber has a 950 gallon/hour submersible pump.Water returns through a u.v. and back to the pond via a venturi.I have about 20 Koi 4 to 10 inches long. Up until about 2 weeks ago the filter seemed to work perfectly, weekly tests reading fine, then I lost 2 fish, both with dropsy.
Tests showed high nitrite, frequent water changes seem to be bringing things back to normal.The fish however, don't seem at all happy. I think Imay have carried on feeding when the water was too cold, the filter "shut down" from the cold and perhaps the waste was not recycled. I would very much appreciate your views on this matter.
My kids love to feed and watch their fish, it really worries them that the fish have nothing to eat.Therefore Ihave decided to heat the pond, please could you tell me where I can buy the heat exchanger etc. I look forward to hearing from you.
Ian - UK

Dear Ian,
The general construction of your pond seems excellent and similar to that recommended in my pond construction articles. You do not mention aeration, but I assume the matting is very well aerated with a large air pump such as a Hi-Blow 40 or 80.
Japanese matting takes up to two years to mature fully, and as the pond was only started in August this year then you have introduced too many fish too quickly. You do not say how many fish are 8-10 inches long, or what size of fish have died.
I would not consider putting four inch fish outside in an unheated pond as late as August even if the pond was matured. This size of fish are usually very stressed and really require a warmer environment during their first winter in this country.
It is generally a good idea to stop feeding the fish when the temperature starts to drop down to about 50 degrees F, and reduce to once a day before that. That is why these conditions are not suitable for four inch fish.
Larger fish do not suffer by having nothing to eat for the winter, but there is some concern that our winters in the UK are too long for the fish. There are also some that think it is not natural to heat the pond.
If your children enjoy feeding the fish then their enjoyment will be extended for at least ten months of the year if the pond is heated in the winter to say 55 degrees.
Most of the bigger koi dealers sell the stainless steel heat exchangers and thermostats, and several are advertised in the Koi Carp Magazine.
The other parts for the boiler etc can be obtained from a heating supplier. Eric.