All animals poop. And pee. If you intend on keeping an animal indoors, you should also include in your intentions the possibility of a smell. That said, fish won’t stink up your house. Also, the female “Fancy Mouse”, unlike some other mice, has “scentless urine” and goes through cat-like lengths to keep herself well groomed. But, again, you need to clean the cage out weekly. Same with the fish and cleaning out the water. All, repeat, ALL animals smell.