large acrylic tank on slightly bowed stand

[Cermet]

For my 75 gal stand project I took a hollow fully flat "door' assemblty (very cheap at homedepot), I uniformly spaced three added three tubular aluminum rails (3/4" by 3/4" and 4' long) on the inside of this hollow internal 'box' structure (the rails ran along the length of each side and one ran down the middle) and this formed the top of my 'cabinet' for the aquarium. The end of 'door' that I opened had this piece reglued back into place. The top looks professional and better still, easily supported the full weight of the aquarium in a uniform manner.

You could use floor fill to level the top of your stand and then place a similar reinforced support so even if the top warps some, the metal tubular rails will carry the load. Do not depend on just filler - water can damage it and it is brittle (i.e. breaks and cracks in time under load.)

[Brhino]

From the description of the problem the center is bowing down making each end high.

With 3000 pounds of force pressing the sheet of plywood down onto the sheet that is not uniform the over lay sheet would be difficult to keep from just forming to the same shape. This puts us back to square one on the problem.

In between the top and bottom sheets of plywood will be a set of wooden furring strips that are precision cut to main the proper gap between bottom concave plywood and top flat plywood. The furring strips will sit directly on top of the joists and posts that hold up the bottom plywood. Essentially, I am negating the bottom plywood entirely without removing it.

I've actually been advised against the use of floor fill for this application because it is rigid and brittle, so if there is any flex of tank or stand the fill will snap and fail.

[genocidex]

I am for the planning of the surface to make it flat as well. If you have a router tool maybe you can use it to shave the decking down to a level surface.

[Brhino]

well, it is 1/2" plywood, and the gap is a maximum of 1/4", so that would work in theory... I don't have the tools to do that though. Might be able to borrow something. Hmm. That would be the cleanest solution.

[Cermet]

Here's a possible and very simple solution (not terribly cheap but worth the cost): build a temporary boarder around the entire top that extends 3/4 inch above the highest point (the ends) to form a couter dam. Fill the entire top with liquid polyurethane, which will self level very well as long as the stand is FULLY level. Allow to harden for a few days. Remove the counter dam strip.

This dried polyurethane surface will provide a level, extremely waterproof and ridged platform on top of the existing plywood. Many tables are made this way and the stuff is durable and strong.

[Cliff]

If I were in your situation, I would get a 1" thick pc of plywood cut to fit and shim it across the whole top of the stand. The ugly shims that can can been seen from the front can later be covered with some nice trim.

Use at least a 4 foot long level to check for level.

IMO, your only other option is to build another stand using straight pcs of 2X6s. You can find them with almost no crown and then straighten out what remaining bow there is in the wood when you attach your up-right supports to the 2X6s

[4Mike]

You might try Marine Grade Plywood. No flaws or large gaps in layers. Better than Veneer

[Longshot]

I used to work in a saw mill and in my experience ALL lumber crowns one way or the other. My father is a general contractor and I learned from him to always crown your lumber UP when building. Under a load it will straighten. If it bows down it is in theory weaker than bowed up. In my opinion rebuilding it with the crown up is the best option. Even the shims will crush down in time with that much load on it. I used 2x4's for my 150 gallon, I stood one upright and screwed the other flat to the top of it so that it looks like a "T". I did this for the top ring and the bottom ring. The uprights were screwed together like an "L". And the outside sheathing is made of 1x8 pine boards. this thing is strong enough to hold up a pickup truck. I put the crowns up and when I placed the 400 Lb tank on it , she leveled right out without so much as a creak from the wood. If it were MY stand I would redo it for peace of mind.