40mm Heatsink and Ball Bearing Fan $12.00 (cheaper if you can find without the fan)
ARCTIC COOLING SLIM SILENT PRO $7.99
Thermal Grease Compound $1.00
(no pic... i'm lazy)
Cheapo HOB Filter... Maybe you have this on hand... if not $15.00
12V ~4amp power supply (laptop transformer) @ surplus store $15.00
Total cost not including shipping is... $60.00
If you are like me, you have some of this stuff already. Look in your phone books for electronic surplus and give them a try. (if you live in northern california, look up HSC in Rohnert Park) You may be able to get some of the heatsinks and fans cheaper there. I estimate that I will purchase the aquaclear HOB at petsmart for $21.00 - 15% hook-up. So All in all, you may be out as much as $60 but i think it'll make a quality cooler.
On to the design.
We'll have to cut out the 40x40mm square for the heatsink in the back or side of the HOB. I will probably set up a guide and use my dremmel. If you got a 40x40mm with a fan... take the fan off. Get some 2 part epoxy from HD and let it sit in hot h2o. Cut the tip off and depress plunger (try to get like a dixie cup or something small to put it in. You may actually want to buy 2 syringes of epoxy to ensure that you can "dunk" the heatsink. Also, if you don't have some, get some aquarium silicone.
So we dunked our heatsink and hung it upside-down so that the excess epoxy will run off the prongs (haven't figured out how to suspend it though). Let dry and cure (heating prolongs dry and cure times). Ok, so it is cured. Go and slap that sucker in the hole you made on HOB and see how it looks.
After that, plug in your peltier for only a few seconds to figure out the hot and cold side. Stick the epoxy'd heatsink onto the cold side of the peltier with your thermal paste. Stick the other heatsink (can be and probably should be larger than the 40x40mm surface that the peltier unit is) to the peltier with the paste. Apply a little aquarium silicone to all 4 sides of the "peltier sandwich." Let it tack up then place assembly into hole on HOB. Apply more silicone to create a watertite seal (hold in place with masking tape... or gravity).
Ok... for the math and theory...
A 37 Watt Peltier is suppose to do 85 BTU. British Thermal Units. The quantity of thermal energy required to raise one pound of water at its maximum density, 1 degree F. One BTU is equivalent to .293 watt hours. So if we are making 85 pounds of water change 1 degree, we have selected the proper peltier. If you need to cool down a 5 gallon tank 5 degrees... 5 gal x 8 lb/gal = 40lb X 5 deg = 200 BTU. We wouldn't have a big enough peltier device. However, for my application, i have a 2.5 with a ton of LR and sand so it only holds about 2 gal. My max cooling would be 2 gal x 8 lb/gal = 16 85/16=~5 deg of change.
So the big question would have to be... what size is your tank? I would assume anything over 4 gallons in a hot environment (like my apartment) could not feesably use a peltier system unless you got a huge one and had a gignormous powersource. However, their are many other things to consider. Could you set your AC (if you have it) at like 80 so that the tank would be working with your ambient temp, not against it? Could you consider spending the money and making a low volt LED custom hood (ive been reading up on those... very interesting). Other considerations would be to find out how much your pumps, PH, etc are affecting your temp. I'm sure their are some things you could do to test this out (Qbomb comes to mind but it scares me at the same time).
Ok, now please remember that i have not tried this out yet. I have little dinero at the moment. it'll take some time to get this together then even longer to put it together. Please share some thoughts about design and (sigh) design flaws. Anything would be appreciative.