Do you have a lid and how are you filtering, as this would effect the best heater type?
I used celotex board sandwiched between polycarb sheets for my lid, helped with temp so much well worth doing. I also insulated the sides and base of pond with insulation board. I tried one of the online pond heaters but didn't find them as good as normal tank heaters, I keep my heaters in the filter as the water then passes over them which I find gives better results.
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Well I'm heating 2000 litres with 30th heaters but I also heat the room
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Many oppurtunities. Portable heater, normal radiator.. what I think would be practical would be good insulation - thermostat to keep constant temp.