Originally Posted by Brookster123
Why not the top Wy?
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In this case, assuming the top 2x4 are completely level and transitions are smooth, and that this is a standard glass tank, placing plywood on the top serves no purpose. It adds nothing to the structural integrity of the stand. Adding plywood to the sides and the back will prevent the stand from folding up or falling over, although with the L-shaped leg supports that isn't really necessary in this case either. It also adds nothing support-wise to the tank. The bottom black lip of the tank is the only part that makes contact with the stand, so all that extra wood under the tank is really just wasted. All that of course changes if the tank is acrylic. Additionally plywood, if not properly sealed, is far more likely to absorb moisture and swell than those 2 x 4's are, and that is more likely to twist the tank and break a seal or a pane of glass.
The truth of the matter is that this particular stand is way over kill for a tank of this size (60 gallons). But generally speaking, the current trend in DIY stands today as a friend of mine says, "is to build stands that two elephants could fornicate on top of, without collapsing." This tank would be adequately supported with nothing more than a piece of 3/4" plywood stood-on-end under each end, provided you had a way to ensure the plywood would stay perfectly upright. All that is really necessary is to provide support under the black plastic rim of the tank.