One of the cover layers is the bubblewrap like swimming pool solar blanket.
It does an excellent job of insulating due to all the dead air spaces. For
a swimming pool, they lay the blanket directly on the water, which provides
solar heat and prevents evaporation, which is a cooling force. For a koi
pond, you really cannot cover 100% of the surface due to the need for gas
exchange, and if it doesn't cover all, then there is an area that is
radiating heat from the pool. There is a solar heater that is marketed for
ponds. It is a black plastic device with many tubes for the water to flow
through. It uses a pump in the pool to supply the water through the tubes.
The pump needs to be put on a timer, since it will act as a heat sink at
night. On cold cloudy days it will give off more heat than it collects. I
really don't have a good place to install one of these, though my local pond
supplier has used one on her pond in the spring to give it an early start.
--
RichToyBox
http://www.geocities.com/richtoybox/pondintro.html


"Iain Miller" wrote in message
...

As a thought, have you investigated some kind of Solar Panel to help? My
father has heated his swimming pool (in the UK!) with these for years &
they
have paid for themselves several times over.

I.