we just had to replace all of ours.
most of them were about a year old and started cannibalizing every litter.
we started right from the beginning, with 12.4
took FOREVER to get them grown up, and OF COURSE as soon as they were starting to breed, 2 of the colonies killed a female. and then i kept a baby and grew it up to replace the killed females, and one of them died too!
grrrr. i was going to have 1 tub with "replacements" growing up at all times, but since i decided to have 4 colonies instead of 3, i don't really know how i'm going to swing that yet.
we only have 6 tubs; 4 breeding colonies, one grow-up bin, and one bin for cleaning rotation.
i'll get it figured out i suppose, lol. MOAR RACKS!? hah, i wish.
but yeah it's definitely a good idea to replace them sometimes, because they sure do go downhill after awhile.