If you want a quality aro then buying from a reputible farm and making sure it has a cert and chip would be advisable. Otherwise just buy a cheap RTG from the markets, just don't expect it to be an XB or high-grade. If you buy a cheap aro then that's most likely what you'll get.
I think if you really want to build your tank "elevated" (which IMO isn't a good idea for a large tank, but each to their own) then building a ply/timber tank and lining it with fibreglass or something similar would be a better idea than concrete. If you want a 3d background you could carve one out of polystyrene and epoxy/paint over that. Have a look for threads by a user called VLDesign, he has good plans for building big tanks on stands up off the ground.