The last time I used a small little bookshelf thing for a 10G I put a "stabilizer bar" across the back of it. Just a strip of 1x2 wood that I painted black and attached on the upper right and lower left of the bookshelf/table. It really added a lot of strength, and you couldn't even see it after the table was fully laden.
For a 20L tank, I think the rule of thumb is if you can sit on the table with no issues, then the tank should have no issues. Although I guess that assumes you're a hefty 200lb guy like myself, hah. A full 20G tank is roughly 160-170lbs.
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