With something as small as a 20, I wouldn't worry too much about it. Go for what's on sale. Obviously check to make sure that there are no cracks, no breaks in the seals, and that the joints are even. Thickness should be standard. All Glass Aquariums makes a good tank, so does Visio (for the small ones). Find one you like, and let us know... I'm sure we'd be able to help you out more.
HTH Got my 20L for $3 at a yard sale
~~Rob
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