I had a lengthy (and somewhat snarky) response about the generational lack of science being taught and applied to common problems in favor of teaching the new science of opinion. I deleted it...
In short, we live in a world were opinion is used daily to trump fact and drive consumer and political habits, with this generations nit-wits becoming the next generations (literal) teachers. A society full of folks not taught the fundamental skills needed to invent or survive without the help of governments. Bleh...
From science class in 6th grade (I am 43, so maybe they don't teach science anymore)... 1 BTU equals the amount of energy needed to heat or cool 1 pound of water 1 degree Fahrenheit.
If we look at our 100 gallon aquarium that we want to maintain at 80F and observe that it rises to 86F over the 10 hours that the lights are on, then we know that the tank has absorbed about 5,000 BTUs of energy. (100 Gallons* 8.33 Pounds * 6D = 4998 BTUs)
If we know (for example) that the well water is 65F, we can cool the tank water with it. If we coil up a bunch of tubing and toss it in the sump, then run well water through it we can transfer the heat from the tank water into the well water.
How much heat needs to be transferred? Well 5,000 BTUs in 10 hours, or roughly 8.5 BTUs per minute to maintain the water at 80F.
The temperature differential between the tank target temperature (80) and the well water (65) is 15 degrees. If we pump .25 gallons per minute of well water through the heat exchanger tubing coiled up in the sump, that would mean we have a potential heat exchange of around 124 BTUs per minute. You see .25 gallons of water weighs about 2 pounds. The Delta T is 15 degrees, so that 2 pounds of water has the potential to do 30 BTUs of work. We only need around 8.5 BTUs right? So working backward 8.5/2 = 4.25. That means if we can get the water leaving the heat exchanger to be 4.25 degrees F warmer (69.25F) then we can keep the tank at a constant temperature.
What does that all mean? Simple enough. You simply need to make the heat exchanger at least that efficient and then control it with a temperature controller.
How do you make the exchanger that efficient? You make the material more heat conductive or you make the contact area longer or you make the contact time longer, or any combination of the above.
You are limited in materials (I will leave this to others willing to contribute) but you can change the length and the flow rate of coolant. Clearly, in this case we want to minimize the use of coolant, so the longer the contact time and the larger the contact area the better....
Hope that gives you a start Zachts (and others).
Of course, there are already dozens of heat exchanger formulas and methods to calculate what is needed, but with the simple information above, one can use trial and error to arrive at a reasonable and efficient result
So if you need to maintain the temperature in a 100 gallon tank that gains 2 degrees in an hour from its desired set-point...
