It really depends on the livestock you are planning on keeping. Many tropical fish don't do well at cooler temps, but some fish can tolerate low 70's to high 60's. Research the ideal conditions for your livestock and determine what temps will keep them happy.
If you keep your house at that temp all the time that's what the tank will be. The aquarium water will pretty much track with the air temp. I have a thermometer in the tank and one that measures air temp and they pretty much read the same. If your fish like those temps you may not need a heater.
I have a tank in my front window with black substrate. The heater is set at 78, kicks on at 76. The sun warms the tank to 82 during the day if the blinds are all the way open. Do you have an internal or external thermo?