I am building a hydronic heat system that heats my greenhouse right now. I have the benefit of having a wood burning furnace that I am installing custom made heat coils into the firebox which will heat the fish tanks, as well as the air in the greenhouse via radiators. I am certain that coils within a compost pile would work very well for heating the fish tank water in a greenhouse. The main problem to contend with beyond the composting itself is finding a fish safe coil. The system would be much more efficient if the actual tank water flowed through the coils(heat exchanger) than with a closed system with two heat exchangers. Copper is your best bet, but there is the toxicity issue. Stainless would be ideal, but is very expensive, and difficult to bend. If you did a closed system, it could eliminate the toxicity problems, but would take a much larger pile to create the desired temps. I wonder about Badflash's idea, the Babington burner. Could you use this as a waste oil burner inside of a drum, or woodstove, with some stainless steel coils? The intense heat would require a shorter tube length than a compost pile, and make it doable. With enough water as thermal mass, this may only need to be fired a few times a day to achieve an adequate temp. http://www.hilkoil.com/product.htm I am using two coils similar to the largest one pictured on this page, but they are both quite a bit larger than this. Each one has nearly 14 feet of tubing inside the firebox. I am running an open system with the actual tank water going through the stainless coils, supplying the radiators, and when the temp drops too much in the fish tanks, a solenoid valve will divert flow directly into the tanks until the thermostat is satisfied.