Outdoor heated ponds are a impossible dream here in England, our cold wet weather makes heating difficult to say the least. It takes a 2000w heater to heat my 18x8x10 fish room which is insulated on all sides, to heat a 20x10x4' (thats a shade under 6000 gallons) pond will require around 14960 watts of heating at ambient room temperatures, to combat the chilling effects of rain, wind and snow you can expect to tripple that, i was going to figure the cost of the heating bill alone but the PFK website is down so i'll get back to that. What is needed is a building errected over the pond which is kept at a constant 28c, a outbuilding of this size requires planning permission from the local authority and needs to be built to current building regulations and passed by the local authority buildings inspector before it is legal.