Styrofoam

glass (framed or rimless) or acrylic.

I think its fine under acrylic as long as you use the correct stuff. Framed glass tanks are a no, and not really sure about rimless glass tanks. - Do those still have a bottom frame on them usually?

 

I think it's usually recommended not to. Just make sure the stand is nice and flat and the rim is touching especially at all the corners.

 

Foam under a rimless glass tank: yes, and use the white beaded styrofoam (1st choice) as it compresses easily and that is what you want. the blue/pink foam board is a second choice and is OK too. The point of the foam is to remove any variances in flatness of the stand surface. You should still use 2x layers of 3/4" thick high quality plywood (Aruco brand is my choice, being made again) on top of your stand, with shims between the stand and the bottom layer of plywood to level it.

Foam under an acrylic tank: this is generally a no-no. Foam will compress and will do so unevenly due to the lesser rigidity of acrylic, meaning it will compress unevenly and this will cause the bottom panel to bow upwards or downwards in the middle (depending on your stand) and this will cause stress at the seams and could lead to tank failure. Acrylic is much more forgiving so the 2x layers of Aruco is fine, shimmed flat from underneath, and wiped smooth to get rid of any granules of anything, but acrylic will not crack on a point source like glass will which is why you don't need the foam like you do with glass. The exception I have been told is to use neoprene because it compresses and doesn't stay compressed permanently, but it's expensive. And not necessary. If you feel you must use foam, use blue or pink board and go as thin as you can to minimize compression. White board should never be used under acrylic, it compresses too much.

Foam under a rimmed glass tank: I'll fall short of saying never and say avoid unless it's an absolute last resort and then only under certain conditions. Glass rimmed tanks are designed to be supported by the frame only. For tank under 48" long, you only technically need to support the tank by the corners, and then 6" from each corner. Longer than 48" need full contact support all around but that can be done with shims (which will also compress). When shimming I usually shim from both sides so that the 2 shims make a flat surface but that's me, I'm pretty picky. I use cedar shakes from Menards for all shimming (much thinner, and smell fresh). The only exception to this is if you want to make a thin foam frame right under the foot of the tank (contact part of the rim only) and then you would want thin white styrofoam, as thin as you can get, so that it compresses permanently and usually evenly under the tank. But even then, if you make/install the stand right (base in full contact with floor when unloaded/no tank) and it's level and true) you should not need to use foam.

Never, EVER place an entire sheet of any kind of foam under a rimmed glass tank. Ever. If you have and haven't had a problem, then you got lucky. Foam in contact with any other part of the bottom of the tank other than than the foot of the frame will push up on the frame crossbrace and/or glass progressively as you load the tank and can very easily cause the bottom to crack out. For many tanks, this is a tempered panel and they don't react well to pressure like this. For untempered panels, it's just as bad except you might have more than 2 seconds before your tank is fully empty after it cracks.

 

Rimmed glass tank is going to be what you get from Aqueon, Marineland, etc. It will have a sturdy trim piece with crossbraces and places to put lids and such.

If the tank is rimless, it is likely custom built by Glass Cages or Miracles or Reef Savvy, etc. These will typically have a piece of angle trim around the bottom edge even though it's "rimless" but that is more for appearance and keeping silicone joints clean looking versus support. That kind of trim will be very thin, like 1/16", and usually just an "L" shape. You still put that kind of tank on foam. I had one made by Miracles for Grove & Platt and that is how it came. They told me to just use the white styro that was in the crate and set it on that.

you can see the trim in this shot



and here a bit

 

Aruco brand is what I use. 23/32", 4 orange stripes on the end (might be 3, can't remember for sure).

I've read glued together and screwed to stand but if you think about it, you put plywood on then tank on top, check it and shim, so you would have to take the tank off again then glue & screw. IMO PITA. Weight should keep it in place