I ordered the bracing materials today. The tank does not have any play moving left to right because the plywood is screwed into the metal at several points, so the main play is forward and back (obviously the worst type).
The bracing will be the same steel material, with opposite diagonal bracing on the sides and then a /\/ shape across the three in back.
The next trick will be getting into and more importantly out of the tank. Roughly 2 feet between the top of the tank and the ceiling, so I know I can slide in, but I am more worried about getting out. I think I have a way worked out with suction cups as steps..but I'm still working on that point.