I strongly recommend Anna Edey's book "Solviva" which is about greenhouse food-growing and living in New England. Super inspiring with many practical ideas. Her commercial greenhouse was heated by chickens but mostly by thermal mass. Her home greenhouse is heated by thermal mass and a small woodstove.
I am not attempting to undertake this project until I can figure out my costs. Heating a greenhouse seems to be a costly undertaking and could kill the profitability of an Aquaponics system. I am looking for ideas that can be implemented as well as calculated for a business plan. Thanks for the book suggestion!
Ok then, a rocket mass heater is a good, cheap solution to the problem. Paul has lots of good RMH resources here on permies. I'll leave the exercise of discovery to you. another possibility is to use the earth (or geothermal) to heat the greenhouse.
Use a backhoe and dig a ditch 8 feet deep. It would be best if you made it pretty long. maybe fifty feet or more. The larger the greenhouse, the longer you need to make your ditch. Of course it has to circle back to the green house. place an 18 inch culvert pipe in the bottom, seal it up. At the low spot, place a sump pump. Have both ends of the pipe come up into the greenhouse. (One inlet on the north side, the other on the south.) use a fan to blow air through it. a reference to this idea is here. Of course this is pertaining to a house, but most factors still apply. http://static.monolithic.com/plan-design/airpiping/index.html This can be used to cool the greenhouse in the summer as well.
But, of the two methods, a RMH is the cheaper option. You would have to work hard to spend $2000 to build a RMH, and I suspect you could do it for under $200 if you are good at scrounging. Fuel cost for a RMH, under $100 / yr, assumming that you don't need to keep the greenhouse above 65 degrees at night..
Other great choice (if you have room) is to put a couple of compost piles in the greenhouse. They produce a good amount of heat and will give you a good heat base...Oh, and compost of course.