the rack looks nice, but aside from the ventillation issues that have already been mentioned those tubs are really small. something about the size of a 10 gallon tank is really the smallest you want to go for either a pair or for one mom with babies. they do need a little room to move around and get away from each other. plus, once you have a mom with 12+ babies that are close to weanling size they will be stacked on top of eachother in one of those tubs and the amount of urine will be a lot more than something that size can handle.

maybe you could re-work the rack for 10 gallon tanks? i'm not sure that there are any tubs that would fit in that depth of a rack and still be big enough.