There are 4 quarts in a gallon, so 40 qts is 10 gallons. Tubs are short, so there is a lot more surface area in a 6" high 41 qt. tub than there is in a 10 gallon aquarium. I keep babies up to 400g in 6 qt tubs, 400g - 1,000g are in 12 qt tubs and 1,000g+ are in 28 qt tubs. Once my females start getting over 2,500g or so I'll be making a new rack for larger tubs. in gallons thats babies in 1.5 gal, yearlings in 3 gal and adults in 7 gallon tubs.

Edit, Ed got it.