I used a very cheap observatory. I bought a standard 7 ft x 5ft shed and didn't fix the roof on. I merely tied the roof down at each corner with a stout elastic luggage strap,to a strong peg in the ground, put 4 bits of wood on the walls to rest the roof on. For added peace of mind I put on top of the roof 4 large bricks.
With a rectangular shed I was able to put in shelves - how would you do that with a round tank??
Inside, all I did for the tripod was to cut 3 holes in the lawn (about 200mm dia and deep) and filled them with concrete. Just before it set, I plonked the tripod on top to make a little dent in each bit of concrete, and then when set, I aligned and levelled the scope and BINGO there was my fixed scope. For observing, it took me about 3 minutes to remove the bricks, and undo the elastic straps and lift the roof off.
Modifying a poly tank sounds very dodgy to me - you are going to destroy all the strength of the cylinder, it will become uncontrollably floppy and you will have to spend a fortune beefing it up - if at all possible.
