errrr yes. Sorry I did mean to say "outside" but I mean the "inside of the outside walls" and .... oh, stow it, I'm just digging a deeper hole here, let me start over.

I kind of just meant the perimeter instead of a cross brace across the centre of the tank. But yes, the bracing is on the inside.

I suppose there's no reason to not have bracing on the outside but 1) the tank will look ugly and 2) I don't know if that's still referred to as "euro" bracing.