I would not cut the leg off just yet I would allow the leg to dry up even though that sounds risky I would just give the leg some time. You have an American bullfrog and a 20 gallon long is more than enough for an injured frog. The temperature should be cooler the temperature you have it at now is good and because these frogs live in water the humidity is not something to necessarily worry about. You just have to keep the Frog really moist remember it's a pond frog. The frog would probably benefit from a UVB light 10 to 12 hours a day. I hope you're planning on keeping this frog because if you release it it will probably become prey for something.

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