The tank is 2ft long, about 56l. I've attached a photo so you can see the stand it's on, which is an oak table with four legs instead of a full base, and a solid top which has shown no signs of warping. The tank is glass, on a raised edge so the glass bottom doesn't make contact with the wood, and there is nothing underneath (which I believe is correct for a raised edge). It's on a laminate floor and has been filled for almost a year, and had been used before by me but not for more than a few months.
I've always known the level was a bit off, but as I fill it up so the water surface isn't visible and clearly pay very little attention while doing water changes, I've only recently become concerned about how off it is. What's mostly made me anxious is that the seals look white to me (but looking at a photo from setup, I think they always were). I thought I could see one of them starting to peel inside, but I poked them with a credit card and I think the seal just has a chunky edge instead of being smoothed in line with the glass. The back left corner is about 1cm lower than the front right corner, so it's off in every direction.
(In my teens at my mum's house we had a 3ft Juwel tank on a stand, on an uneven carpet floor and it was over 1cm out. It took several years but one day I saw water slowly dripping from one of the corners. I panicked and called a LFS who just told me to empty the tank asap. I didn't have another tank set up so I put a smaller one together right away, and because of the leak we transferred the fish after a few days. Obviously it didn't have enough time to cycle and we lost some of the fish. After emptying the tank we never used it again, it's still empty at her house. I suppose the trauma has stayed with me.)
I can't find much info about leveling a tank on a table with legs instead of a solid base. To anyone who has experience of this, can I remove most of the water and slide something like a piece of cardboard/foam under the back left leg? Would I have to shim more than one leg otherwise risk not all legs being in contact with the floor? Is the tank a risk that should be emptied, never used again and stock donated to a LFS? I'm a very anxious person so I feel I need to ask how much I need to worry about this. So many thanks in advance for any advice
I'm probably worrying over nothing and tend to read too much into things (when I set up the tank I asked four people for opinions on the safety of extension cables; every opinion was "I don't know, I've never thought about it" 
). Looking at an old photo I can see that the imperfections I can see in the seals and the glass not being completely flush were there from the start, so aren't evidence of warping.
