You really need to get a heater. Your fish cannot live in such low temperatures permanently. The temperature is so low that heater pads, pre-set heaters, and other non-adjustable heaters won't do you any good, so make sure you get a 25 watt heater with an adjustable temperature dial like this one: http://www.fosterandsmithaquatics.co...m?pcatid=11368
Until you get a heater, you can put the fish in the warmest, most stable part of your house away from drafty windows, doors, direct sunlight, vents, fans, electronics, etc. If you have a space heater, you can put the fish in a small room, like the bathroom, and set the space heater to heat the room to 80 degrees. If the space heater does not have a thermostat, you should choose a larger room. Never put the fish tank close to sources of heat like lamps, vents, or space heaters because it will cause the temperature to change too dramatically.
If there is no way you can afford to get a heater or other necessary items for your fish soon, return him to the pet store or find someone else who has the means to take care of him properly.