Cant help you on the heater but no you cant float a cover on the surface. It will prevent the natural exchange of gases including the absorbtion of oxygen. Also your Koi wont like it! If you suspend a cover the layer of air you trap beneath a cover becomes warmer than the air temperture and helps to insulate the water further. Also it must allow the sun rays through but keep the cold air out. You realley want a cover with Solar properties like the one made by Aquosis but they are expensive. They do work I have one myself. Failing this good quality bubble wrap, preferably double with a 50mm air gap in between the layers. The problem with bubble wrap is you have to make a frame to hold it etc. A good temperature to go for would be 15C but this will require good insulation.
Posted on Dec 13, 2006, 7:15 AM from IP address 86.140.85.44