er2murp2, can't help you with the tilapia source there but I would at the very least suggest you consider a Summer fish (Tilapia), Winter fish (Trout) scheme. Having to heat your tank might make Tilapia unprofitable any time other than Summer. You probably already considered this since you asked about sumerable heaters but thought I should throw it out there just in case.
20 x 40 is a good sized greenhouse. I'm not sure but it sounds like you are looking for 6400 to 8000 CFM of ventilation.
This publication might help http://www.extension.org/pages/32535/greenhouse-ventilation. Your location is probably warmer than here. Some greenhouses have vertical sidewalls with polycarbonate panels that can be replaced with insect screen using screen window kits from the big box stores. I've also seen inflatable side walls on greenhouses so when you want to let air in just deflate that portion of the side wall http - http://www.usgr.com/ventilation/poly-vent_inflatable_side_vents.php
You might also want to extend the greenhouse in the future? Some people start with a 20 x 40 and expand it to 20 x 80 or more as they grow their operation. With natural ventilation it might be more difficult to do side by side greenhouses later on so how big you want to get might be important.
Hope this helps.
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