Green tree frogs
One adult GTF needs at least a 10 gallon enclosure. For each additional GTF, you need to add at least 5 gallons. For example: 10 for 1, 15 for 2, 20 for 3, etc. These are minimums and bigger is always better. Young GTF froglets should be kept in a "bare" 10 gallon enclosure with background on 3 sides, paper substrate, a water bowl and a few large river rocks until they reach 1 and 1/2 inches in length. At that time, a plant, corkbark, and limb or driftwood piece can be added. When they have reached 2 inches in length, they can be moved to a larger fully set up enclosure.
You will need a secure enclosure of appropriate size for the species and number of inhabitants. Most commonly, store bought aquariums or critter keepers with screen lids which securely attatch are used. Custom enclosures, using a combination of glass/plexiglass and screen, can also be made. Keep in mind that bigger is always better.