Constant temp. in tank

[Daniel Budfuloski]

  Don't know if this would work (but it is what I plan on doing with my system when I get it up and running).  I'm going to have a sump tank burried 4-6 feet down (where the temperature stays at a constant 50-55 degrees and has the effect of thermal cooling/heating) and have it pumped up to the surface of my fish tank that is burried half in the ground.  Then i'm going to siphon the water off the bottom of my fish tank to the grow beds.  My theory is...  with constant 50-55 degree water pumped to the top of my fish tank and then being pulled to the bottom by the siphon effect, it will keep my whole tank the same temperature year round.  Do you guys think it will work?  Oh, and one last thing...  I live in Arizona where in the summer, temps can reach 115 and in the winter, night temps can hit freezing... 

[Jonathan Kadish NYC AA Chair]

You will only have cooling/warming from the outside surface area of you sump so the amount of BTU heat transfer will be limited to that surface area. You can do the calculations for this by determining that surface area.

You will have much better surface area & heat transfer if you bury a few hundred feet if pex tube and figure out a way to create a heat exchanger in your sump by creating a closed loop.. You wouldn't want to pump your fish effluent through that loop because you will get biological build up. Over time in a season it will become less effective because the ground warms or cools as it exchanges the heat with the sump.

[Randy]

Using the ground as a heat sink will help but it seems to me that solar heat gain/loss from the grow beds will still cause your temps to fluctuate.

[Benjamin Frimmer]

Hey Dan - cool idea...I too thought the same thing!  Here's what I have found over the last 2 years...

The temperature of your water will almost always be the equilibrium between the ground temperature and the average nighttime low.  For instance if your ground is 50, and the nighttime low is 70, your water will be about 60.  The same is true for the winter.  

So controlling the air temperature that comes in contact with your surface water is the priority here.  A greenhouse is best.  Hoophouse/cold frame next best.  Insulating your grow beds and fish tank above ground will also greatly help.  Shading your fish tank in the summer will help as well. 

I guess the real question is what kind of fish do you want to have?  If you don't want to spend the money to build a greenhouse or insulate everything, then maybe go with Koi / Goldfish.  Those things are indestructable!  If you're trying to grow edible fish like Tilapia, then you have to find a way to keep the water temp at least 55 or they will die.

Anyways, hope you can learn from my mistakes.  Good luck and let us know how it comes along!