Ply alone will flex, but an inch in front, inch in back, you MAY be OK since you have trim, maybe.
I believe those tanks are meant for load at each corner, more than across whole span of trim.
For me, I'd do the 2x's and ply top, but I'm a carpenter, easy for me to do.
Judgement call on your end, but I personally would sleep better if it had full support.
120G mixed reef, 17G planted