Muske;4593904; said:Greenhouse idea will help retain heat.
A simple heater using a black gardenhose and pump works surprizingly well. Lay the hose out on the ground, or better the roof of a shed, and allow the pump to cirulate water through the hose into the pond. Fairly slow flow and not running it at night will do a good job and cost pennies to operate....
Muske;4597208; said:I would think it is safe.
Black Poly irrigation pipe could probably be used as well. Not as flexible, but turns can be made with glued fittings.