Eventually we can heat the whole globe! Oh, wait..

Last week, we've moved our stuff in the basement (big basement!) of somebody who needed the heat for the winter (we are in Canada). It's been 1 week, and we didn't started the heater once with a temperature between 0 and -10 Celsius outside.

And usually, that house pays around 3500$ for heating in a year.

Right now, I really believe it is possible to reduce of the heating cost down to 0$. I'm curious to know how it will be if we get temperatures of -30 Celsius, I hope we have a mild winter 

If you have a water cooling system you could use a thermostatic valve to divert the hot water to either aquarium or a radiator.

One way to get an idea of how much heat your aquarium needs is to plug it into a kill-a-watt.  My guess is that an aquarium doesn't lose that much heat.  Likely the output of your rig is vastly oversized compared to the needs of the aquarium.  Now 500 gallon, or a backyard swimming pool that might be more like it.

My rig is only 2 5770.
It's my gaming rig, so by the time the fluid got from my rig to the aquarium (one room over) there would be heat loss out of the tubing.
 
It was just a silly idea I had, but if the aquarium was in my same room, I'd look into it more seriously, but might give an idea to someone else reading.