here something for you been there done that my rtcs and ripsaw and shovel nose are in a 12'x6'x4' pond with a 500 gallon side sump 2 fx5's i got off ebay for $100 each new uv sitr and in size the tank is a 5000 gallon laguna pond filter and just two 300 watt heaters to keep a nice temp of 79F. ill post the link of my pond build as soon as i find it any questions just ask.http://www.monsterfishkeepers.com/forums/showthread.php?397872-my-1700-gallon-indoor-pond&highlight=
What is your room temperature. Do you think 2 300 watt heaters would work for a 12x6x3 outdoor pond or maybe 7x7x3.
I live in Canada so the air temperature is usually 22-26