I find it is easier to keep 2 sets of 10 rats in 2 10gallon tanks than keep 20 in a 20L. Clean-up is much easier. I have 6 10-gallon tanks for my 1.2 breeding colony. 1 tank for the breeding group and 5 tanks for the grow-outs. I have to wait a while to feed the grow-outs because I have 3 adult bps who need big rats and like llovelace said, they take a long time to grow - about 3-4 months in my case.

I bought 7 10-gallon tanks from craigslist for 20 bucks!