Hmmm, like the others said, looks like a chiton. I bet you have two of them - they simply can't double in size in a week, but one hiding while the other is out cruising around is a distinct possibility!
Jay Hemdal
p.s. - after I posted, I had another thought. If you are absolutely positive that you only have one chiton in the tank, some species can appear to grow larger and smaller (at least widthwise) based on how much of the soft "girdle" expands around the side of the plates.