Hey storm. Welcome! Ironically Im from vernon as well. Interesting!

I am a bit concerned that you mentioned a frog that will "not eat your other frogs"... are you housing them together!?

I would HIGHLY recomend a GREY TREE FROG.
Which is actually native to our home state of connecticut and is also not protected.
Very Friendly. Very talkative. Handleable. NON AGGRESSIVE.
Will hang out on your hand and stare out you.
Is capable of surviving freezing of their internal body fluids to temperatures as low as 17.6F (so its a little more owner friendly if your heater breaks)
Possible hand feed after training.

Whites tree frog would be my second recomendation. NON AGRESSIVE / NOT POISONOUS
Will eat smaller frogs such as your green tree frog. Grow quite large and will need a larger tank than 20 gallons tall in a short period of time.
Not as talkative as the grey tree frog. Will hand/ tong feed.
CONS: IS PRONE TO OVERFEEDING. POO SMELLS BAD!!

Another concern:
-Is this a standard 20 gallon? You will need a TALLER tank for a tree frog. Not a standard 20 gallon long thats meant to be an aquarium for example, 24x18x24 exoterra (whites) or a 18x18x24 for your grey.

If that is your extra tank I would highly recomend you keep that as a spare quarantine tank verses getting another frog just because you have it laying around.

-GreenTree