Hey Pete,
Man you are creeping me out!!! I just thought this up the other day and have been asking a couple others about it.
Here are some links I found:
http://www-2.cs.cmu.edu/~cap/raid/chillers/diy/
http://www.webcom.com/thansen/GlacierBBS/AquariumChillerForum/
http://www.coolworksinc.com/coolproducts.htm
http://www.boquetriver.org/adoptopta2.html
http://saltaquarium.about.com/cs/chillerssetup/
http://www.atlantisaquatic.com/DIY.html
The main disabvantage I can see is that commercial ones cost so friggin' much. The home made ones look like they might work.
Here is a in the works plan I have on how it might be set up. Everyone feel free to offer your suggestions or comments:
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100' of flexable plastic tubing, max 1/2" diameter. Run this out of the tank, into the fridge, tons of loops in the fridge, run out and back to the tank. Attach one of those table top fountain pumps to the setup and fill it all with water so that there is always around 4" of water in the bottom of the tank (I am thinking a small turn over time would be better, maybe 1 gallon every 5 minutes. The water would be inthe fridge longer so cool more and, conversly, be in the tank longer so absorb more heat.) Have the outbound tube at the bottom of the tank to suck the water out but put the inbound tube up top and have it cut in such a way that the chilled water flows down the back side of the tank. I figure this will help cool the tank a little at least. If more cooling is needed then an airstone can be added, the bubbles will be cooled and thereby cool the air (I have also thought about having the water flow out onto a sponge-like material and having a fan blow through/over that, again the result is cool air.)
From everything I have read a setup like this can cool a 75 gallon tank full of water to 70 degrees so I figure in a smaller tank (say 20 gallons) it should be possible to drop it to 50 with little trouble.
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Ram has already mentioned to me that the fountain pump might be too weak so a real aquarium pump might be needed.
Anyone else?
Pyro