Heater Efficency?

The gold fish will be fine but the plecoc need to have the water temp at around 78 deg or they will freeze and die. The waters temp will only remain stable for a couple of hours and and then it will start to cool down to room temp. If you have a good heater like a Hagen tronic or a aquarium systems stealth heater than you will be fine if you go on vacation and leave the heater on.

Well, goldfish are coldwater fish. They don't need heat and will be fine for a week in 40F.

Your pleco, though, will need heater. Can you give him to a friend while you're away?

I would not trust a fishtank heater to cope with sub 40F temperatures.

Otherwise, can you find some way to heat just that room? Like a small space heater plugged in? The pleco should be okay for a week at around 65F, so if you can keep the temp in that room at 60F, plus the water heater, it would be okay.

it depends on the size of the heater. I would invest in another heater and have 2 in the tank which should be sufficient to keep the tank warm enough.

I assume they are going to turn down the heat, not shut it off as if it gets to -40 their pipes will freeze in the house.

So assuming the house will be at around 50 degrees two heaters should take care of heating the tank.

The pleco will certainly die in that low of temp. Take it to a fish store and donate it. If it is a common pleco it shouldn't be in such a small tank anyway so you will be doing it a favor. The goldfish will be fine in low temps, they are a coldwater fish anyway. Also depending on the type of goldfish it probably won't work in there either. Fancy goldfish can possibly do great in a 20 gallon tank but only with heavy filtration and you have to be religious with water changes.

Buy a generator and plug the heater into there. Buy a fish heater and plug it in =) Also, if it was a little warmer, goldfish like tempuratures at 50-70 degrees =)