I've been debating setting up some tubing with a heat exchanger (sealed) to provide heating to my aquarium, since I need to heat it anyways. I just haven't got around to it. Plus I don't know how hot it will get (55 gallons), so I'll need to do some testing first so I don't come home one day to fish soup!
If it works, then I'd keep the regular heater on to provide backup in the event that my GPUs stop mining.
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.