Hey there, Cosmiccow. Why does that name sound like I have read it somewhere before in a story? I do not have a concrete pond. I cannot imagine that you are not going to want to make it larger like everyone else. I have to admit it would frustrate the kind of obsession most of us have to make it bigger.
Since the river (which is moving and is a lot more water than a 720 gal. pond)does occasionally freeze over, I would look for a deicer as well as something to prevent the cracks concrete ponds (and swimming pools) are prone to. There are heaters available but the cost of running them is prohibitive unless you are independently rich. They are made to keep stock tanks from freezing and I don't think they raise it as far as 55oF, but I could be wrong. How about looking into solar heaters? I used to heat our above ground swimming pool that way. Sandy

Since you're planning to do natives I would think you would want it bigger myself. Also am wondering why you want to keep it above 55 if you plan on natives? 55 is a magical temp regarding a lot of tropical fish, that's when they will turn belly up, your pond doesn't need to be that warm. A bit of reference for you...you are quite a bit colder than I am and my pond will be below 55 for several months during winter- you won't be trying to heat it during cold spells but during the entire winter.

A deicer won't keep your pond that warm, it's not a heater. Like Sandy suggested, you are looking for either a soalr heater or one of the large heaters that are on the market these days that use a lot of elcetricity.

Thanks for the replies. I think the solar pool heater should work fine. I plan on keeping small species of natives. Mostly minnows and pygmy sunfish. I might increase the size to about 1000 gallons. Also, 55* F is a bit high, but I wouldn't mind keeping some semi-tropicals.
Cosmic Cow was the puppet and comic book heroine in "Too Close for Comfort" sitcom in the 80's.

Thanks all...