Introducing new rats to a colony

So I have been breeding rats for a while now and some of my breeder females are getting a bit old. I want to keep my 1:4 ratio up but I am not sure if I should just create a new colony or put more females in the old colony. If I where to add more females to the colony I was wondering if I should just put them in or is there a special procedure.

thanks,
John

I would keep any females from current litters and raise them to breeding size and use them to replace your old ones. That was you don't have to deal with quarantining new females or having them fight with your current rats.

One word QUARENTINE, just like in snakes quarantine is essential new rats should always be quarantined especially if you have a healthy and strong colony.

Thank you both for your info I will be sure to quarantine before adding them to the colony.