Compounds of 魚心

• 魚心
  (n) favor done for somebody
• 魚心あれば水心
  (exp) if you do a favor for somebody, you will get a return favor (lit: if fish is friendly toward water, water will be kind to fish, too.)
  mutual back-scratching
• 魚心有れば水心有り
  (exp) (id) Scratch my back and I'll scratch yours