Your ferret should always have a large, clean, comfortable, ferret specific cage or "kitty condo" type cage available to keep him or her safe when you are not at home or in the event of an illness or emergency. Ferrets should be caged or kept in a "ferret proofed" area of your home when you are not home to supervise them and at night when you are asleep. They should be kept indoors in a temperature controlled environment if outdoor temperatures are hotter than 80 degrees F and colder than 45 degrees F. Ferrets should never be housed in a glass fish aquarium due to lack of air circulation and moisture buildup on the glass bottom. This excess moisture can lead to bacterial and fungal infections on the skin and in the lungs (can lead to death).
Fortunately, there are a variety of fun ferret cages to choose from. You can spend any where from seventy-five to one thousand dollars on a ferret cage. My advice is to start with a multilevel cage that has "easy cleaning" features. I own two ferret cages - one with a plastic bottom and coated wire that easily lifts out for cleaning, and one with a metal bottom that is easy to clean as well. Cages with metal bottoms may have the potential to rust and corrode if exposed to urine, feces or water. However, if you purchase food and water dishes that attach to the cage, litter boxes and throw rugs to cover the bottom of the cage, rust shouldn't be too much of a problem.
If you are purchasing a cage with coated wire, keep an eye on your ferrets to make sure they are not chewing the coating off of the wire. This can lead to a gastrointestinal blockage or poisoning. Also, make sure your ferret cannot fit through the slats of the cage.
It is important to note that you should never line the bottom of your ferret's cage with cedar or pine chips. Some research has found that Pine and cedar chips contain oils that may be very harmful to your ferret's respiratory system (and liver). You will often see ferrets housed in a large aquarium with cedar chips in the pet store. Please be aware that this is wrong and you should not house your ferret in that fashion. Rather, use a ferret specific cage and cover the bottom of the cage and any wire mesh with a throw rug or soft (and durable) pieces of fabric. Ferrets should not walk on wire grating alone. Put a corner litter pan on one side of the cage and food and water on the opposite side of the cage. This will encourage them to use the litter pans. Ferrets will generally not use the bathroom around their food.
For more great tips on how to set up your ferret's cage and much more, check out The EverythingFerret Complete Guide to Ferrets and Ferret Care.