A water-tight aquarium is the best frog cage and should be no smaller that 20 gallons.  It is important to have an escape proof screened lid to prevent runaways.  The aquarium should be set-up with 1/2 land and 1/2 water.  

Just as their name implies, tree frogs live in trees most of the time but do require dechlorinated water for their skins health and to eliminate body waste.  Therefore, for the easiest care, it is  recommended you use a large rock pool bowl.  This water should be cleaned daily to keep your frog at optimum health. 

Since tree frogs are excellent climbers and live in the trees, providing branches is important.  Be sure that you only use branches that have been cleaned and sterilized.  Do not bring in branches from outside because these can carry bacteria and bugs which can be harmful to your frog.  

Fake plants work best because they are easy to clean and care for.  If you feel your frog needs a "natural" environment, try air plants in the fish section.  Keep air plants very moist and away from any heat sources. 

The temperature in your frog cage should always be between 72 degrees and 85 degrees.  A Fluorescent light will help in keeping the temperature even. 

You can also use an undertank heater.  Be sure to clean the cage once per week with hot water and a good scrubbing.  Do not use soap or cleaning chemicals, as this will harm your frog.

Ventilation:
If you have frogs that live entirely underwater, such as African Dwarf frogs or African Clawed frogs, make sure they can't get out of their tank.  However, frogs need well ventilated tanks, particularly frogs that need greenhouse-like environments. The moisture in the tank can lead to fungal and bacterial infections. This type of tank should have a screen cover.
 

Overcrowding:
Overcrowding a tank with too many frogs will cause the frogs to be unhappy.  An overcrowded tank is breeding grounds for bacteria and fungi. It will also contribute to injuries, since the close proximity will likely cause frogs to run into each other or objects in the tank, or provoke fighting amongst the frogs.  In addition, some species of frogs release chemicals from their bodies that are toxic to other species of frogs or some frogs will eat other frogs.  For this reason, you shouldn't mix breeds in the same tank.