Hello and welcome to FF

! Sorry your local pet shop put you in this situation. Can you talk to them and return some of the frogs for $ or store credit?
You could section off the 55G; but in reality, only to help you get through emergency. What are the actual species we are discussing? In general terms, each dart frog should have a square foot area; so the smallest footprint can recommend for 2-3 frogs is an 18x18 in. ExoTerra or similar. Even if they are the smaller thumbnails

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IRT the height; if planing a planted vivarium you will loose around 6 in. to drainage/false bottom and substrate layers. So if dealing with a very terrestrial species, can barely scrape by with an 18 in. tall tank; but your plant choice will be very limited or you can forget live plants and decorate with artificial ones. For more arboreal dart climbers (i.e.
Ranitomeya); 24 in. height is pretty much mandatory. Hope this does not discourage you to find a viable solution. Sadly, even the two 12x12x18 in. ExoTerra's you have are not suitable for dart frogs

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