Crews were called after the animal was found trapped in an old water tank on Tuesday.
Lorraine March from Kent Wildlife Rescue Service was at the scene, in Chatham, Kent.
She said: "We have no idea how long the fox was in there for but it was slowly drowning.
"The work the firefighters did was amazing, they spent 40 minutes working on it.
"They called the RSPCA who said they were available but suggested the crew took it to the nearest vet to have it put down.
"They got hold of us instead and one of our volunteers was there inside 20 minutes."
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