The guinea pig cage should have good ventilation, aquariums are not a good choice for a Guinea pig cage. Fresh air is a must for any of Gods creation even the Guinea pig. Wire bottom cages should not be used, the Guinea pig could get his feet caught in the mesh causing the lose of a limb or worse.
Choose a cage were the Guinea pig can move about freely, it doesn't have to be huge like some sites try to tell you, I personally choose to use Large Rubbermaid type tubes for my piggies, you can purchase them at Walmart at a very reasonable price. If you own a Cat or Dog the tub probably isn't a good idea being they will probably torment the piggies or worse. Guinea pigs need daily exercise and in most cases their cage is not big enough to satisfy this need. Playpen fences can be bought or built to make a special play area. You can purchase playpens from Petsmart or just about any quality pet store.
Another option is to give the guinea pigs free range of a room in your home. This is the choice that I have choosen, my piggies are allowed to run the whole room. They run about chasing one another and bouncing constantly. Petsmart and other stores do sell proper cages especially designed for the Guinea pig, whatever you choose to use for your cage always think safety first.