The above ground grow beds are quite the heat/cold exchanger. My tank is in the ground and I like it that way but the temp of tank water fluctuates.
Wouldn't the ground also act as a heat/cold exchanger...Like when the the water is warmer than the surrounding ground...and the FT is not insulated (with say some polystyrene or whatever)? I mean heat always seems to move from hot to cold right?
Yeah, the above ground grow beds have been a pretty big cold sink, that's for sure...especially running them flood and drain during the cold winter. It's probably cool (no pun intended) to run them constant flood in the winter when the surrounding air is real cold. Seems like it would help with that cold sink effect, especially if you are heating water (which I'm not)...
I have my 200 gallon tank buried in the ground. The premise of putting things in the ground is based on stability of heat/cold or thermal conservation of energy. Since I live in south Alabama, the mass surrounding the fish tank keeps the water from getting too hot too quickly in summer when days average in the 90's and nights might only dip back into the high 70's (fahrenheit) and the coldest days of winter might get down into the 30's.
In reality though, an inground fish tank does not benefit from this conservation of heat at optimal efficiency because most fish tanks (like mine) are just not buried deep enough. Even down here in the south, the real benefit of burying something is not realized until about 4 to 6 feet underground, but every little bit helps, right?
All that being said, I agree with Dy here with insulation being very important. The ground, because of its mass is very efficient at heating up or suck heat out of your tank water. I have three layers of insulation surrounding my tank. between the rubber liner and the structure is polystyrene about one half inch thick. Then, I put standard rolled insulation within the wood framing (like a house) and another polystyrene layer on the outside against the plank covering.
Since the outside air fluctuates so much where I live, this helps keep the water temp more stable.
One other thing to consider is that it is easier (or more energy efficient) to heat water than to cool it. I raise tilapia in my tank so I try to keep it just above 80 degrees in there. Last winter, I struggled to keep it above 70 and went through two heaters.
But, to finally answer your question, yes, the ground will suck heat from your tank faster than air if there is a substantial temperature difference and the tank is not well insulated. The benefit really comes in the winter time when you are heating a tank from a stabilized temperature of 50-60 degrees rather than the temperature of the surround air, which cold be below freezing. If you have an open backyard system as I currently do, then putting the tank in the ground and creating little greenhouse covers for your growbeds will help in the winter. If you have your system in a greenhouse and the greenhouse is heated, then it is not as beneficial to bury your tank. In fact, in a heated greenhouse, an exposed tank (still need insulation though) may actually help stabilize temperatures and add humidity to the greenhouse.