You will notice blue lid tubs in the 32qt racks, those are 15 qt rubbermaid tubs, I love them, cause with the lids on, they can be kept in the 32qt rack.
The other 15qt tubs are sterlite ones, and they do not fit in the racks, that's why they are stacked on the top shelf of the hatchling rack w/ heat tape tacked to the wall for back heat.