Dunno if you remember, but last summer I modified an AC to fit into my tank (see my sig to a link about it). I've now decided to try something else using an office water cooler. Essentially it's the same idea as Martin Hingst, except that I used a water cooler instead of a fridge. Should save me a bit more on my electric bill as the AC compressor was a 1/4th HP and this water cooler is only 1/12th . So far the results are promising. The cooler easily got the water down to 4c while circulating. Anyway, here's some pics:
The Rad. I got a XSPC double 120mm rad. My tank is about 4x4x2 so it should be good enough to cool the whole thing.
the fans: I got two ultra kaze 130 or so CFM, but I'm lowering the voltage on the power supply so it's not like a crazy tornado inside the tank. I used these last year on my AC setup.
Here's the water cooler. I removed the taps on the front and mounted some hose barbs to it. Below the hoses you can see the external temp control unit for the water.
In the water cooler tank: I ran a tube from one of the tap outputs to the water pump. I used a basic aquarium water pump. You can also see the probe for the temperature controller.
Only thing left for me to do it attach the rad and fill up the cooler. I'll post again once I have to start using it. But with the preliminary tests I've done, it looks like it'll be good .