I might have a look in the shop where they are building these tanks.
Are they setting up on a bare floor?
If they build the tank warped, say, 1cm "clockwise" and your stand warps 1cm opposite, then the total stress could be much worse than imagined.
The stress increases rapidly.
If there is a 1cm warp and it goes to 2cm because of the stand, the stress doesn't just double. It rises geometricaly, like 4x.
Only it varies over the whole tank, depending on the shape and construction.
MASSIVE DISCLAIMER: My largest tanks to date are only 55 gals...
I do not like the idea of foam pads. They can protect the tank from contact with irregularities, which is important. But they do not provide equal pressure at all points of contact unless you have a perfectly flat stand.
I think the best way to do this is to set the tank in a bed of setting compound, then add a little water for weight and let it dry.
There are many things you can use, but in every case you want to wax the bottom of the tank, or use 2mil plastic as a release sheet, so the tank doesn't stick to the stand. I also use bedding compound between the stand and the tile floor after shimming things up level, so I would get solid contact there as well. If your stand has leveling feet you don't need to do this, but it needs to have several feet so you distribute the stress on your floor and the stand. I think if you have a 300 gallon tank on a stand with only four or 6 feet, that is a big mistake in normal houses. You want one foot for every 20 gals, minimum, depending on the floor. I have 4 inch thick concrete slab floors, but I would not start putting 200 PSI in discrete loads.
Thanks for both of your input -- wow I didn't really know that about foam as pretty much every shop here in Thailand makes and sends out all their tanks with foam pads and 1/4" sheets of wood, including the shop that made this and my other 325 (they also put the plastic trim on both tanks as pretty much all manufacturers do with 99.99% of tanks here -- I've seen very very few rimless tanks in Thailand; acrylic tanks are likewise very rare).If the tank has plastic trim, most manufacturers will void warranty if one uses foam. It does not act as a level if the tank has plastic trim. Just so that's clear, foam should only be used when the tank is custom made, with a flat rimless bottom.
Thanks for the further great tips Ulu, and hope others can benefit from the info as well -- can you come to my house this weekend?? I'm having a BBQIf the glass was clean and the silicone was good, the tank should be usable; IF the stand is solid, and you get good EVEN contact on all the plastic rim. A little out of shape or a little out of square or a little out of plumb isn't important. Every tank is.
BUT full solid contact at the rim is critical on a rimmed tank, and zero contact elsewhere is the general rule. When I did my first aquarium stand, I used Wood Dough. It didn't stick to the plastic, did stick to the wood, and has excellent bearing pressure. I bedded the tank when the dough was still wet enough to flow slightly. That was 30 years ago. It's been drained and moved 5 times, and I scraped and resealed the inside of the tank a while back though it wasn't leaking.
My stand doesn't have a full sheet of wood. It has a perimeter maple frame with birch plywood sides.
I have stands with full tops, but they are nominal 2" thick birch, and I flat-sand them with a big board to avoid setting beds.
It takes hours and hours, but it will outlive me.
This is tough to do on a steel frame. I might shoot an even bead of thick sealant on the steel.
Then set the pre-waxed tank, fill 2" of water and check the level. 30 min DAP paintable caulk will work. It will flow for 10 mins while you set things, and you can fill the tank 30 mins later.
If the top of the stand is very irregular, you can put a dozen 6mm to 8mm thick shims down, set the waxed tank rim on them, add 2" water, adjust shims as required 'til perfectly level, then shoot or trowel the bedding in. When it sets, pull the shims and infill the holes with the same compound. If you use black and do it neatly, nobody will notice.
Some tank stands just support the ends and at a few points along the front and back. It works but a cheapo way to do things.
Finally, you can silicone the glass tank right to the stand, waxed or unwaxed. Later you can cut it loose with a guitar string or fishing line.
Just observe the curing times before you fill more than 1" or 2" for leveling purposes. Do not try to level a filled tank except in emergency. Always let out lots of water first to reduce dangerous localized stresses.