Oh, and you don't need a fancy thermometer.

I use LED lighting (it's actually an LED shop light just set on top of the glass tank cover) and I don't get direct sun, so there isn't anything that raises up the temperature except the ambient air. When I do a water change I always check the water temp and try to keep the new water within 1 degree of the current water. Are they both fine at 70 degrees? Pretty much!

Could a Mexican Hairless survive sleeping out in the falling snow? uh, no. Keep in mind that saying 'goldfish' is like saying 'dog.' Can a husky survive sleeping out in the falling snow? Heck yeah. It's old language that basically means they're not 'tropical' fish that can die if the temps dip below 70. Goldfish are not true 'cold water fish' in the way that something like a trout or a char. On the low end though, goldfish can survive in ponds and the like by going torpid in near-freezing temperatures. I like to keep a lid on my tank to keep evaporation down (and stuff out), which also helps keep the temperature more stable. If the room is a comfortable temperature for you, it's fine for them as well. Consider providing adequate water parameters for the proper growth of your fish. However, pond goldfish can thrive up to a temperature of 40F by hibernating themselves at the bottom of the pond. The really important thing is that you keep the temperature from fluctuating too much too fast. 68 to 74F is the ideal water temperature required for the proper growth of goldfish. When we had a heat wave and my AC broke, I was monitoring the tank and ready to put in frozen bottles of water if it got over 83. A lot of fancy goldfish are bred in places like Thailand where they put on jackets if it gets to 70 degrees and closer to 110 is the norm. I don't use a heater or anything, I just have a warm home. I have a 75g tank that is regularly 73-80 degrees.

Temperature for goldfish aquarium