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How big do most tree frogs get, particularly red eyed tree frogs?
RETFs grow to around 2 in. for males and 2.5 in. for females. Tree Frogs come in all sizes from tiny to big. Largest is the White Lipped Tree Frog with females reaching 5 in. or so.
Will my cage be big enough?
Yes.
Do they need water to swim in or will they do fine with consistent misting to keep up the humidity?
Tree frogs do not need to "swim"; but the enclosure should have a water dish where frog can soak filled to frogs chin level with dechlorinated tap water.
Misting system or fogger? Or both?
Either one, your choice. I just mist by hand.
Do I need to simulate wet and dry seasons?
No. Some frogs do require some conditioning if breeding is desired.
Can I have multiple frogs in the same cage? If so, how many?
Yes if same species. Quantity depends on the frog species.
Can I mix species of tree frogs in the same cage? Say green tree frogs and the red eyed tree frog?
Not recommended.
Could I put poison dart frogs in the same cage?
Not recommended.
Do they handle well?
Frogs are basically look but don't touch pets. White's are not too stressed by limited handling once aclimated.
Any information on lighting, heating, and planting is appreciated too!
It all depends on frog species. There are some tree frog care sheets and a guide to help select a frog in this section:
Frog Forum - Care Articles. Also, the vivarium section has many nice builds.