Actually i can think of a very cheap way to keep an aquarium cool- use your cold tapwater supply(usually between 8-18 celsisus depending on season) to cool it down by running the aquarium water through a titanium heat exchanger that instead of using gas and electricity simply uses a solenoid to open a valve from your home coldwater supply so the heat from your aquarium is transferred to the coldwater supply that runs to your geysers... this also means the geysers would receive slightly warmer water and thus need less electricity to hit their correct temp ... right now the theory is all in my head but soon one day im going to make this work!
Good theory, but a bit impractical.
The reason I say this is:
1 - there would probably be less than 10C difference in the tap water and the tank water, this means you would have to have a heat exchanger the same size as the tank to be able to transfer the energy efficiently, otherwise it you would have to run the water for hours to get any effect.
2 - if connected to the geyser, it would only work when someone uses hot water in the house, however it could be connected to the main supply to the house, so when any tap opened it would cycle though the exchanger, but basically "unreliable"
3 - Costs, I haven't done the calculations, but I really doubt it would work out cheaper, just roughly, a chiller would use about 0.5kWh to chiller a 1000l tank by 1C costing about R0.40, it would cost approx about R2.10 using about 250ltrs of water. (like I said, just a quick sum, not exact figures)
PS, colder tap water would help the efficiency, however, doubt the tank would get hot if its so cold outside anyway.
ice and a beer perhaps?
