Hopefully this isn't too off course, but heavy metals are not detoxified by Prime. If these are a huge concern, you need a carbon block.
Heating new water with old water sounds like too much of a pain. I guess you could put a smaller barrel in a larger barrel, fill the smaller barrel with new tap and then fill the larger barrel with the old water? I can heat 110 gallons of water with 500w in about 4 hours (~$0.22), but I only do this during the harsh AZ winters.
Also, depending on how much water you change at a time, is heating even necessary? A short trip into the 70's isn't going to harm a healthy fish if the water is aged/pH stable.