JMartin got it down cold. 4" is plenty for the size tubs we're talking about. I bought a Jason's Jungle rack that uses 16 & 32 qt Sterilites and they are all doing fine in them so far. Have the juvies in the 16s (2 per shelf) and the adults in the 32s. I have three others in aquariums, which is what I started them out in, and am seriously looking at buying another rack to move them all into tubs. Right now am waithing to see if I get any clutches this winter before I buy anything. If I do, will be getting a hatchling rack from someone, and put off the next juvy/adult rack for a little while.