I work w/wood alot and the problem w/plywood is that it warps and separates easily if it gets damp, then will get moldy & rot under the aquarium. If you have solid wood framing, add a couple layers of deck sealer (waterproofing) on all surfaces under the aquarium that may get wet. "Green" wood will also crack slightly as it dries out and any moisture will alter the sturdiness unless you've treated it. It sounds like a pain, but a couple of coats only takes afew hours to dry enough to seal and not be sticky. The framing you describe should be sufficient to not have to install plywood, it'll just be a hassle. sorry I didn't see the photo of the stand till now, Unless that plywood is part of the unit, I would still just use the frame.