If you are going with the concrete slab, it should be at least 100mm thick, and 200mm bigger than the tank all round, to avoid cracking at the edges.

The mesh should be 52 grade, and the concrete a minimum of 20 MpA, poured on a solid base of 50mm compacted crushed rock or packing sand.

When preparing the slab, box up formwork, with pegs or stakes every 500mm to avoid movement.

Want something to really set your job off, you can have a sheetmetal box formed and welded in one piece.  This can even be done with 100mm x 40mm C section purlin.  This is the ultimate in a tank installation project.

The most important adice I can give is, make sure your boxing or metal surround is set up dead level.  Nothing looks worse than a 1.8 or 2.0 metre tank out of level.