If it was me... I would definately have it poured concrete. It only makes sense... to me. Most MFK's , I think built plywood tanks because they are putting it into a space that was prebuilt...
Think of how you want it done and size etc.. you have this done...
try and draw up the best of your ability some sketches...
Have an engineer look at your stuff and edit your blueprints for your house.
When the home builders put up the forms to pour your foundation they will build the forms for your tank... it will be poured and done just like that.
If you go to sell your house down the road it will either add value and the next owner will love it... or you could take it all down and take it with you... then they could turn the tank into a storage area or something.
You would basically be paying for the labor hours to build the forms needed... then the yards of concrete, plus have Zero worries of it holding...
If you built it out of plywood etc... you would have an immense amount of time... that would take months and months. All the plywood and lumber etc...
I would think you would be a light year ahead if you just had it poured....
Also instead of having it in the middle, perhaps place it along an exterior wall.. it would save alot of money and make things easier... in every way... also during the design process you could make a room off to the side for the filtration etc..
Just my .02