That is an awful long span across the middle that does not have any support. The middle of that would only have to bow a tiny bit before bad things happen.
If you really really have to have this basic design to hold a 10g in the middle, here's what you can do. Bring the interior cabinet sides in as far as you can, leaving a minimal width to slide the 10g in. Next run a center support 2x6 the length so you have front, center, and back support along the length of the 55g. Then use no less than 3/4" plywood as the top.