Well, he is 6" across. Maybe 4" tall. Of course, he is my clam, and I am allowed a little inflation as we are dealing with water
So, the mantle is, of course, above the floor of the tank. It is a bare bottom tank, so, the 4" is about right, and the 20" from the mantle is about right.
And for the record, my 90 is 24" deep.