I considered the coil in the refrigerator trick discussed above but decided against it because I am interested in salt water more than fresh water. As mentioned above, if the coils are plastic the heat transfer is not really enough, and if metal coils are used with salt water, you are asking for trouble.
There are several good "in-line" chillers that can move salt water and chill it with temperature regulation. These usually cost about $250 or more, but the prices seem to be coming down. Shop around and report it here! These chillers have only plastic in contact with the water and so salt water is fine.
The Coolworks Iceprobe (Ice Probe) 50W chiller cannot move water, but it can chill, tolerate salt, and has no moving parts except for a small fan. It uses thermoelectric technology to turn electricity directly into cooling. The fan and some bulky cooling fins go outside the tank and a 3" by 1" diameter probe goes through a gasket and extends into the tank or filter or sump. If you already have a pump moving water through your filter you could drill a 1.25" hole into it and insert the probe. Others have done this and posted their efforts on other forums. I took a slightly different approach.
I keep tide pool fish and invertebrates in two one-gallon glass jars. Such jars store things like pickles or olives in stores that sell large sizes for about $6. Such jars are 6" diameter and 10" high. Tide pool fish like water between 55 and 70. Salt water reef fish also need summer cooling in Calif.
Coolworks claims one ice probe can chill 10 gallons by 7 degrees and two probes can chill it 11 degrees. This is overly optimistic. Of course it depends on the flow rate of the water and any evaporative cooling that may be going on in addition.
I built a "cooling tower" 3.5" x 3.5" x 10", open on the top. In it I put two Ice Probes and one Danner #02510 65 GPH 707 mini pump. Ice Probes can be had for just over $100 and the mini pump is about $13. One Ice Probe can be regulated by a controller circuit that Coolworks sells for an extra $60, but considering how cold it actually gets, you don't need the controller.
I drilled two holes, each 1.25" in diameter in two sides of the plastic cooling tower (which I made out of a scrap piece of plastic which I glued with acrylic cement). The two Ice Probes enter at right angles, and cross over, one just above the other. On top of these, I put the pump on its side (the intake vents run vertically and the flow control valve is on the top). If you pack everything in efficiently, you can cover it all with about 6" of water in the cooling tower. A plastic nut on the inside presses against a rubber gasket on the outside to make a water-tight seal where the probe enters.
Although the pump runs at 65 gph with no head of water, my set-up is too small for that kind of flow. Therefore, I took the 3/8" i.d. output of the pump and tapered it to 1/8" i.d. This results in about 11 gph flow. The water level in jar 1 is 8.25". It is connected by a 3/8" i.d. siphon to jar 2 which has a water level of 7.25". This is connected by another 3/8" i.d. siphon to the cooling tower which has a water level of 6.25". Thus, the pump is lifting about a 2" head of water which also slows the flow.
Free with each Ice Probe comes a power converter which changes AC to DC and adds some more bulky equipment to deal with. One fringe benefit is that the circulating salt water adds to the oxygen that I get from my little bubbler charcoal filters. Since the water level in jar 1 is about 1 1/2" from the lip of the jar, I had to cut a plastic lid to keep the fish in and let the siphon tubes out. OK, so what are the stats?
air temp = 78, jar water with pump off = 75 (evaporative cooling), jar water with pump on = 70 (five degree cool down). The few cubic inches of water in the cooling tower get down to about 45 degrees with the pump off. This is rather disappointing considering you have to blow $200 to use dual Ice Probes. It is certainly not what Coolworks says in their literature.
Theory says that if the ambient air temperature is higher, the pull-down should be greater. In summer, I would need about 10 degrees. We shall see. Another factor is that I am using cylindrical glass jars. You probably have a rectangular tank. Also, if your tank is acrylic plastic, it will leak cold much slower than glass. Given the water levels quoted above, the surface area of water in contact with my jars is about 348 square inches. This is roughly the same surface area as a rectangular tank 4" x 7" x 12".
Not impressed? Well, you may be better off with one of the everything-in-line combination pump, chiller, controllers mentioned above. However, you will do better than me if your aquarium is plastic or quarter inch glass as opposed to the standard thickness of glass pickle jars
.
Have fun and good luck,
Boondocks
DIY chiller 
Pretty straightforward idea ... +2 from me !
