The easiest and most effective way to do this is build a greenhouse over the entire pond which can get pricey. It's a lot easier to do that and run some sort of AC unit on the green house for heat to keep the interior ambient air temp in a reasonable range than it is to try and use conventional heaters to heat the water while still exposing it to the cold outside temps. If your 5500 gallon pond is really deep that will make it a lot easier to keep it warm all winter since we don't really have a "frost line" so to speak in Texas and the ground temperature a few feet down maintains a constant 70 or so degrees. Is it cheap no. Is it the best way to heat it, absolutely.Think we coud re-answer this question but for southern texas instead? Temps typically get to 36ish for a week or so, and are below 60 for 2-3 months. Any ideas on how to heat my 5,500g pond for tropical fish?