Tom, let me add my little bit of experience to this as well. I have built two aquarium stands, one for my own family and one for a customer. Ours was a wee little thing knocked together very quickly when the girls came home with goldfish won at a carnival. I made it from oak veneered 3/4" MDF with solid oak edge banding. It had an inset door, and a flat top with just a little bit of a rim to keep the aquarium from sliding around. It was butt joined with biscuits, and was nothing more than a simple box to elevate the tank to vewing height and store supplies.
The second one was a little more elaborate, and I never got pictures of it before I delivered it. It was made from wormy maple and 3/4" birch veneer ply. It was for a salt water 75 gal tank. It had raised panel doors and was pretty much a simple box with raised panel doors. It was glued and screwed using Kreg pocket screws. I splurged and used stainless steel screws since it was a salt water tank.
To reinforce what Jeff wrote, you do not need post and beam style construction with 2x4 lumber. My tank is in storage, but it saw lots of service in its day. The other tank is going strong in place for a year now with no problems. A properly constructed tank with square joints and solid joinery (like pocket screws) is perfectly capable of holding a tank full of water. Square and level is essential, so if the floor where it is going is not level, you need to plan for levelers in your construction.
I hope this helps you out some.
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