I know wooden pallets can support a huge amount of weight, I've loaded six metre long pallets with a couple of tons of Gib board (I think you'd call it sheet rock?), I've also seen them flex by about a foot over less than half their length when you pick them up with a forklift with that sort of weight on them. The 1" timbers between the blocks on the pallets are pretty flimsy and wouldn't do much to support the aquarium evenly. I'd certainly never build a timber stand out of wood like that, especially used flat rather than on edge, and subsequently wouldn't consider using pallets either.