Well I just came from the "animal expo", actually a reptile show/swap but they're trying to branch out and expand the customer base. I noticed that the overcrowded rat tanks. mostly 10 gal, really did smell pretty bad, but they had like 20+ rats in them. My rats never smell like that, but mine have plenty of space, 2 cubic feet or more per rat. And they're in wire cages, which I think makes a really big difference in the lack of odor build-up.