YOU SAID:

When fish are fishing for fish, fish are fishing for fish

INTO JAPANESE

魚が魚を釣っているとき、魚は魚を釣っている

BACK INTO ENGLISH

When a fish is fishing, the fish is fishing

INTO JAPANESE

魚が釣りをしているとき、魚は釣りをしている

BACK INTO ENGLISH

When a fish is fishing, the fish is fishing