What is non toxic to fish, has a low freezing point, and floats on water?
There are oils sold for swimming pools to slow or stop evaporative cooling . That would be a problem for air breathers like mosquito larva. I don't know anything about it because I went with 1&quot; thick Styrofoam panels. That is what I currently have on my pond ( 10' X 5 '), works very well .I cheat a little and have 150 watt aquarium heater in the 700 gallons. My pond is currently 59 F while nearby open water is 49 F. For your tiny area the Styrofoam is fine leave about 1/2&quot; on all sides for air . On the other hand why not do the obvious and put in a 25 watt ( if they make them that small) aquarium thermostat/heater ?