That is prob all right. It is on the edge of a pane, but the seam is still sealed & that is not particularly a load bearing point. I had a chip like that on a 10 & I used it without problems. 20 isn't that big so the water load won't be too high on the seams.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.