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I am working on designing and building a polished concrete table or countertop on which to place DIY glass aquaria (and hang suspended lighting from the ceiling to keep them open-top).

My question is, do I need to put some kind of "cushioning" (ex. sorbothane, rubber, foam) between the bottom glass pane and the concrete slab? Would I risk some kind of structural imbalance or failure by not having such?
 

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Yeah, I'd use the Pink rigid styrofoam insulation. Home Depot Sells it in either 1/2" or 2" thicknesses, from what I've seen. The 1/2" should be adequate & should give you a stable base. This works well for any rough surface or one that isn't perfectly flat & level.
 

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I have often heard of using foam. As a structural engineer I don't know why the foam is thought of as a cure all. I still think the unseen forces are there. Maybe not as bad but I bet it could crack the tank. "Sir Black Hole's" custom stand was bowed (high in the middle). We put the 75 gal tank on the foam and it rocked. Granted that if it was filled with water it would compress the foam a bit, but some of the uneveness and stresses would remain.

I would suggest considering the following:

1) shim the bottom of the tank level. Then cut wooden shims of different thicknesses to fit beneath the tank every 12 inches all the way around the base. Then caulk around the bottom of the tank to hide the shims.

2) Level the four corners of the tank. Use a cementitious non-shrinking grout to fill in the gaps under the tank. You must get the grout completely under the supporting frame for the grout to take the loads. This means getting the grout into the crack one or two inches. Why grout? Hey, the table is made of cement! Why not?

3) if the bottom of the tank is just glass, shim the four corners, take the tank away and glue the four shims to the table. Spread grout between the four shims where the tank will be. Put the tank onto the wet grout with it bearing on the four shims. The weight of the tank should press the excess grout out from under the tank. When the grout dries it will be a perfect bearing surface for the tank.

You might want to wax the bottom glass or use a release agent on it as I don't know if the glass would stick to the grout.

Steve Pituch
 

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I've used foam under many of my tanks and you can see it's usefullness after you drain & move a tank. The tank will settle into the foam as well as the foam will "absorb" any imperfections in the surface. Wherever there is more pressure on the tank, the foam will give way more.

I know it's useful for rough surfaces and slight inperfections in the surface, but it deffinately won't work well for any pronouced bow. Only major problem with it is that it's quite pink.

IMHO, it works, but depends on the application.
 
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