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Corydoras should not be in a 5 gallon. Plus they need groups of at least 6. And they need sand instead of gravel so they don't get hurt or get barbel erosion.
Take back your corys, get a betta and then you can get a frog.
Minimum tank size for corydoras: 20 gallon (long) Except for the dwarf species, none should be in less than a 20 gallon. Find the cory's a better home ASAP and rescue a beta. Then you could *maybe* get an ADF, depending on how aggressive the beta is.