You are at the low end of what would be considered an acceptable temperature, so you will need to act fairly quickly. I would suggest minimizing airflow around the tank to minimize evaporative cooling. Heating up some water bottles and floating them in your sump would also help. Assuming your home is near 70 degrees, your tank shouldn't drop much further.
Here is the question: Why is it tripping the surge protector? A 1500 watt heater on a 120 volt line is pulling 12.5 amps. A circuit should only be run at 80% of its capacity, so a 15 amp (standard home circuit) should max out at 12 amps. That heater alone exceeds what should be run on a 15 amp line, so you should have a 20 amp line run for it. Is that what you have?
I don't know what LFS you have locally, but Manhattan Aquarium, right by the Javitz Center, is pretty nice and seems to have a large service department. I'd bet that they can get you an appropriate heater quickly.
I don't know how Finnex rates, but Amazon could have
this one to your door tomorrow.
I have a 1000 watt element because my basement was COLD until I rebuilt my front porch and put foam insulation over the exterior doors. It was, and is, overkill for my system, but it was there because there were some days where my 500+300 watt elements just couldn't keep up.
Was this your only heater? If it was, and was meeting your heat demand, I would suggest going to a pair of 500-800 watt heaters on separate lines. That will give you some redundancy in the system in case of a heater failure or circuit breaker getting tripped. You have obviously spent a couple bucks on a 640 gallon system. Do you have an Apex or other controller? Mine has bailed me out a few times.
