Chinese Idiom: 如鱼得水

如鱼得水 - rú yú de shuǐ - 
 to perform well in a certain situation, to be in comfortable environment 
(literally means: "to feel like a fish in water")

Examples: 

1. 他兴致勃勃，一举一动都如鱼得水。 
Tā xìngzhì bóbó, yījǔ yīdòng dōu rúyúdéshuǐ. 
He was in high spirits doing everything with happy ease. 

2. 高中生活对他来说如鱼得水。
Gāozhōng shēnghuó duì tā lái shuō rúyúdéshuǐ. 
Lifestyle of a high school student is very comfortable for him.