Rocket Mass Heater with Aquaponics pond

I have an idea to build a rocket mass heater which after running 6" duct through the cob mass, decrease the duct to a 4" which would run inside of cinder blocks that form the wall of my aquaponics pond, before venting out the chimney. I have a ton of questions on my design. Is it possible for this to work with the airflow? Would installing a duct fan in the 4" section be benifitial? What sort of temperatures could I expect to get in the 500 gallon pond? Is it necessary to use firebricks to build the j tube, or could red bricks work? I would desperately love some feedback! Thanks!

check out rob's greenhouse system:
and then work your way backwards - you need to use insulating materials for burn tunnel and riser

Andor aka mechartnik