When I first got my wild caught Eleotris (water cow) in July when it topped off just over 6"
By October it was 9" and thickened up quite nicely,
and again by the coming April 2020, it was over a foot
Granted, these guys aren't very active, mine tended to stay buried in the sand most of the day, until feeding time, dashing out for a couple slabs of fish flesh, but back under the substrate soon after. I kept it in a 180 gal, although I believe any 6 ft tank might have been adequate, at least until it hit 15" or 20".