What size tank (measure please), what is it stocked with, how long has it been running, and what's the make & model of filter you are running? How long do you plan on having the power out for?
if your on tight budget go to your local store walmart etc and buy hand warmers come in small package you activate the heat thing in them there used to put in your work gloves to keep hands warm depending on tank size is ho many u need then take a mason jar put some gravel in it and the warmers tighten lid and put it in ur tank the fish will stay close to it for heat I had power out for 3 days one time and this is what I did and it worked for my cichlids.