Heating a greenhouse / indoor pond
I'm just finishing off building my new greenhouse. It's slightly bigger than my last one so I was thinking of some cheaper/more reliable ways of heating the place (i've had some bad experience with heater failures recently). I'm thinking of putting in a pond to grow some tropical lillies which will have to be heated, but I was wondering whether I could just use the water to heat the whole greenhouse?
I've read the other thread about heating a greenhouse with a hot-tub, I've also heard from another Victoria grower that he used his pond to heat his large stove-house all year (at a temperature of > 20C). The only problem I'm having is thinking of a suitable, cheap and safe way of actually heating the pond. I've heard of people using domestic immersion heaters with an exterior tank which water is pumped through to heat. This sounds quite good but i'd probably need an electrician to come and install it. I've also been looking at stock-tank heaters but there is very little info about these. Any ideas would be much appreciated.
Also, i'm wondering how large the surface area of the pond would have to be in relation to the greenhouse? The rough floor area is 20' x 10', I was thinking of having a pond area of 6' x 10', would this be enough?
Does anyone else heat their ponds? If so, how?