Welcome to MFK! I've been running and troubleshooting pool pond issues for quite some time.
First, do you have any insulation beneath the pond? If so, that will definitely help retain heat.
I recommend installing two to three heaters with a wattage of 250 to 300 watts, all set to maintain a temperature of 32 degrees Celsius. Additionally, cover the pond with large styrofoam or thermocol sheets and insulate the pool walls similarly. Since being above ground and open is what causes the majority of heat loss, this will significantly reduce it.
If all else fails, consider wintering your fish by bringing them indoors to a small but adequate pond or tank until winter passes.
Edit to say, loving your pond and fish. I also have a 500 gallon pool pond, with a pacu, RTC, two Midevils and one Silver Arowana.