About 2" or so is fine. I like to make a slope from the front to the back and side to side. So maybe 1 1/2" in the front left corner and go up to 3-4" in the back right corner. Or something simple like 1 1/2" in the front to 3" in the back. The slope adds some depth and helps your background plants appear larger than the foreground plants, something you'll need in a slim yet tall tank like the 55g.
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