someone in their element; like a fish in water
Idiomatic expression describing a person who is thriving in a familiar or suitable environment, as if they have come alive. The literal image is 'a fish that has found water'.
He is thriving in his new workplace like a fish in water.
久しぶりのステージで、彼女は水を得た魚のようだった。
Back on stage after a long time, she was completely in her element.
A set phrase derived from the literal image of a fish regaining vitality when returned to water. The exact origin is uncertain, but it is a common Japanese idiom.