I think the biggest thing is a pour diet. Don't feed table scraps to the rats, even if they are omnivorous. Get a quality lab rat chow like Mazuri designed for breeding and nursing rodents. Large parrot seed mixes are great for some variety (I find this is consumed completely unlike typical rat seed/nut mixes sold in petstores for rats, which contain alot of alfalfa pellets which rats don't seem to eat at all (or at least its the Last thing eaten).

Dog and cat kibble can be used, but make sure it contains no red dyes (its toxic to snakes and since it ends up stored in the rats, snakes can be poisoned by it when eating the rats).

Overcrowding can cause excessive stress as well. It may be better to move one or two nursing mothers into another bin after her pinks are about a week old.
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PHLdyPayne