Hi Andrew

Do you have your mind set on a tree frog species?

How about P terribilis ? They are very easy to care for.
IMO- easier than red eyes or tiger legs
They are always out and very entertaining
They will do well ( in a trio ) in that size tank.
( you would have to convert the top of the tank to glass - though- it's not difficult)

Pictured here ( on LLLReptile 's site ) - juvenile- Phyllobates terribilis ( 'Mint')

Mint Terrible Arrow Frogs

as they mature the look like this - this pair (?) is about 14 months old
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Have fun deciding!
Keep us posted

Lynn