Requirements: Level 3 Farming
Action: After awhile, it could even be before you purchase level 2 farming, algae will grow in the ocean and the villagers cannot fish anymore. After you reach level 3 farming drop all your masters in the pond and they will catch an algae eating fish then they will carry it to the ocean. It will take 10 algae eating fish so keep bringing the masters back from the ocean to the pond until this puzzle is complete.
Description: "Your scientists confirmed that over fishing of algae-eating fish caused extensive algae growth in the ocean. By introducing a special fish from the diving pool, your tribe restored the natural balance and clean state of the ocean!"