I use 3 sizes of tubs, shoebox, sweater, and under the bed sizes. There's not a perfect weight or age to move them, I really just go by how they look. Though, typically at 1 year to 1.5 years they can be moved up to a sweater tub. And from there 2 to 2.5 years they get moved into a breeder tub. Some of my males and smaller females stay in the sweater tub rack for a while longer.