I haven't added new stock to my current colony in the three or four years that it's been going and I won't either. I've spoken to several rodent breeders who have told me that they've had their colony crash after bringing in new stock.
Any time you bring in new mice, you also risk bringing in the diseases that they may have. I don't know how many generations I've gone through, I don't keep my breeders around for very long, maybe 3-4 months at the most so I'm sure they're fairly well inbred but I haven't noticed any harmful effects. I'd much rather risk a little inbreeding depression then lose the whole colony by bringing in new stock with diseases that my colony may no longer have any immunity against.
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