trying to spec a system for a hightunnel. wondering if tilapa is viable

[Stalemate]

Hello,  I live in 7a grow zone in Maryland.  I'm entusiastic about my 250 gallon system and want to expand extended season growing.  I have an unshakable vision to build poly high tunnel on my property, and devote half of it to a new aquaponics build.  I was hoping to get some opinions on the feasibility of what i've come up with thus far...

 

What i'm considering:

1 foot media bed depth x   4 ' x length of tunnel - 3' walkaround space =

84 cubic foot or 630 gallon grow media space.

Based on this 630 gal grow media space, a ~500 gallon tank seems appropriate, like the one sold through this website.  

I want to raise tilapa.  I'm woried about winter heating.  I ran 2x 200W heaters in my smaller bluegill and catfish setup and 60 degrees often seemd hard to maintain.  I'm thinking that intelligent insulation design did not occur on my part.

I'm basically wondering how foolish this endevaour would be, and any pointers about insulation.

Thanks