I want to do a basement sump someday in the future... but I am waiting for natural gas and forced hot air so I can finish part of the basement to keep heating costs down.
Mine runs 55-65 throughout the year too.
I would also be worried about moisture build up without ventilation.
I have two frag tanks in my basement, avg temp is around 60, I have a total of 180 gallons and use one 800 watt Finnex heater. I did install an exhaust fan in an old dryer vent that is on a timer and has manual switch. I was able to put a slop sink in which makes it convenient to drain water.
The colder the basement the more power you would have to use to keep it at temp. Any loss of power in the winter is going to be felt by your tank pretty quickly. What's your basements temp today?