Sorry for my delay.
I was quite happy being an active member here, but life has actually gotten quite in the way of that, and time reading and responding on this forum had to take the cut for a while, probably more so in the next coming months.
Slab itself is unlevel, either due to settling or shoddy construction. Not level in a "marble is going to roll away from you" kind of way, but enough to pick up with a 6ft level. It's lowest at the lightest corner of the house lol, so that makes me think original work, buuuut who knows, they build on pure clay here since that's all it is with a super high water table, so could be settling as well.
I would guess it’s a mix of both, and one more thing.
clay can wreak havoc on concrete. I had the chemical process explained to me years ago, so I can’t due it justice here, but what I was told is that clay can degrade concrete quite rapidly compared to concrete in other substrates.
clay has the ability to hold onto a lot of water, which concrete will readily absorb, in doesn’t compact very well when is wet and does not do well in the freeze/thacycle. Which paired with shoddy workmanship, high water table will lead to settling. It’s inevitable from what I’ve been taught.
Now that you have identified the problem, the only thing you can do is shim the stand level. You might need to get creative with how you cut/install shins, but basically, any part of the stand that it’s transferring weight to the floor will need to be supported down to the concrete.
Concrete floors that are not meant to be load bearing, sometimes are not poured with rebar to support the structure, so be prepared to shim your shims if that kind of weight starts pushing the floor down some more after the tank is filled. Especially if you’re on clay.
I may be nerding out and going way overboard, but without being that and seeing everything with my own eyes, I’d like be extra cautious with my advice...
*Edited for typos
