I have a rock in my 40B that is about 35-40 pounds. I used a piece of 1/2" styrofoam to set the rock on and covered with substrate. I did it to distribute the weight because my rock has sharp edges and I didn't want all the weight focused on a small spot. Weight wise, I wouldn't worry about it at all. Reef tanks sometimes have a couple hundred pounds of rock in them with no issues.
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