Because I know if it's waterproofed; water, moisture, or humidity can't rot the wood. I have built hundreds of sub boxs for cars. Most have gotten wet. Plywood literley falls apart between layers (even the blue line treated "outdoor"wood). Particle board disininagrates because of it compositon, it soaks up water like a sponge. Mdf seems to hold up the best since it's natural disposition is to repel water for some reason. So if you go the extra mile and waterproof it, it will be fine.
You say that water will get in the wood and rot it, even if it's water proofed. Interesting. I belive there is a multi billion dollar industry that would disagree with you (myself included). As any chemist would tell you, the hydrogen and oxygen molecules can not pass though solid polyurathayne. If osmosis was that strong and efficient, the we would all be turning lead into gold. After all, if ion's and isotopes can pass through easily, why not? Lead and gold are not that far apart. Maybe in value, but not composition.
I only brought up the plastic thing so I wouldn't sound like an a#$hole know it all, but I do know this. polyurathyane WILL stop all water seepage. My tank, my stand, and my floor are are level. I don't see any problem with any type of crack forming due to a uneven weight distrubtation.
(ps. It's actually supported by 4x4's and 2x4's. Check out my homepage and you will see some poor pics of what i'm talking about)
And since it's my tank, it's my choice. That's why.
(And I have several years of expirence at this myself too. I don't just make this up as I go.)
