To clarify, the base of the tank needs to be on a flat surface, so that the tank base is in one plane only, with no twisting, as mentioned above. This is more important than the tank being level. Minor discrepencies in flatness can be alleviated with the use of styrofoam beneath the tank. Styro will not help with levelling.
Since this is a 10, I would add, some of them are made of such thin glass I would be afraid to put water in them. I have one I picked up a few years ago (for free) that is like this.
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