Well, first off, good catch for not filling that. Based upon your pic and description, there is a bow in the middle of the stand, such that all corners are like the pic, correct?
My first move would be to measure the deflection across the entire top using a 2/3 foot level. You may have more deflection near the center than at the corners. Secondly, I'd try to get the stand fixed, whether that means hammering down the center, re welding, grinding down the center, re bracing, whatever.
If fixing the stand does not work (and being welded metal it just might not), I'd probably not use self leveling concrete. I've worked with that before, and maybe it's because it was my first time, but it didn't actually level flat. It leveled 'close' to flat, but not close enough. Then I had to use a concrete grinder to "fix" it. Yikes with all the dust and extra trips. It was ok because it was just my basement and I was going to cover it with tile, but wow, more work than I wanted. Not to mention that unreinforced concrete is prone to cracking, so not sure how long that fix will work.
From the description, you can't actually shim between the floor and tank to fix this issue. It's not that the stand is flat on the top, but tilted; it's simply not flat.
I like the idea of getting a thicker plywood, (1/2" might work, but I'd use at least 1" with something like this), then shimming it. If it's only the edges, and if you shim along the entire edge on all sides, you should be fine. Use good shims (metal?) and as you already said, not just 1 at each corner, but a plentiful number to prop the entire corners on every side.
Do that and re measure the deflection across the plywood and prove to yourself that it's level everywhere across the top. Then fill the tank and enjoy.
My first move would be to measure the deflection across the entire top using a 2/3 foot level. You may have more deflection near the center than at the corners. Secondly, I'd try to get the stand fixed, whether that means hammering down the center, re welding, grinding down the center, re bracing, whatever.
If fixing the stand does not work (and being welded metal it just might not), I'd probably not use self leveling concrete. I've worked with that before, and maybe it's because it was my first time, but it didn't actually level flat. It leveled 'close' to flat, but not close enough. Then I had to use a concrete grinder to "fix" it. Yikes with all the dust and extra trips. It was ok because it was just my basement and I was going to cover it with tile, but wow, more work than I wanted. Not to mention that unreinforced concrete is prone to cracking, so not sure how long that fix will work.
From the description, you can't actually shim between the floor and tank to fix this issue. It's not that the stand is flat on the top, but tilted; it's simply not flat.
I like the idea of getting a thicker plywood, (1/2" might work, but I'd use at least 1" with something like this), then shimming it. If it's only the edges, and if you shim along the entire edge on all sides, you should be fine. Use good shims (metal?) and as you already said, not just 1 at each corner, but a plentiful number to prop the entire corners on every side.
Do that and re measure the deflection across the plywood and prove to yourself that it's level everywhere across the top. Then fill the tank and enjoy.
the pressure is going to be distributed the exact same over these parts.
