One thing i do see is that you give your ferrets a bath way to much. i know that sounds kinda weird but they do. First its not good for their coat and two after ferrets take a bath they do smell worse then they did before(for some reason, I really don’t know, but they do). At first after the bath they smell really good, a day later that is not the case but after another day or so they smell fine again. There is daily spray that you could use on them instead of giving them a bath every week.
I wouldn’t say that the size of the cage would be a problem at all since I had the same size cage once before and I was able to keep down the smell. Another question do your ferrets go in the litter box every time? If they don’t, cleaning the bottom of the cage more then once a week would help. And one more thing, I don’t know what type of litter you use but compressed newspapers help absorb the smell.
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Rose