im looking to build a 600 gallon outdoor pond in ny. the pond will be about 4'6" deep and will be insulated on all sides. in the winter i will place a cover over the pond to help insulate it. my question is how should i heat it to keep it around 75 degrees farenheit. it does get down around 0-15 degrees now and then but usually around 25-30 in winter. How could i heat this with running cost in mind. im open to all solutions thanks.