visualise this (based on a 6x3 footer)
for the base.
yo will need 2 cross beams and 2 side beams. measured and attached to each other at the exact dimensions of the tank ie 6 x 3. perfect fit
repeat this mock up for the... roof..
you will need 4 corner uprights, cut to whatever hight you want the canopy. fix these to the corners of the base and then attach(by turning it upside down) to the other frame
you now need to reinforce the frame by adding support. measure and cut the supports to the length between the base and ..roof
reinforce the corner with 2 uprights per corner add another support in the middle of the side beam and add 2 supports to the front and back
you can now clad the canopy. you can use what ever sheet material you like for the back and sides but i recommend ply for the front.
for the front panel, ideally you want to have it on brackets so you can access the top of the tank.
to ensure the front panel closes you can use magnets or pinch clips paint as req.
on tanks i build, on the panels i paste with car body filer and then prime and paint with car paint to get a PROPER gloss finish. or... i wall paper to fit in with the placement in the room.