I have a glass pen that was originally going to be a coffee table terrarium. I partitioned it into two sections and am currently running two groups 1.2 rats (on each side) the partitions work out to around the size of a 33 gal. Everybody comfortable, everybody producing. Ventilation is no more of an issue then in a cage. They stink either way..
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So one adult rat per sq. foot is pretty good. If you are keeping it for juvi rats you can definitely go more as they tend to pile themselves up in a corner anyhow. I've had 20 (50-65 gr. rat juvis in a breeder cage) and nobody was tripping over anyone else.
Jim