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You have to transfer around 10 or 15 algae eating fish from the pool to the ocean. Not just one or two. So stay on your master farmers. As soon as they drop the fish in the ocean, pluck them up and drop them in the pool again until the puzzle is solved and the algae goes away.

Also, there is a known problem in the game regarding farmer behavior. There is a "patch" that is being beta tested to solve this problem and I would guess it will be available in a week or two.

Clickable HyperLinks:
> VV2 Guides <
> Stew Recipes <
> Puzzle Solutions <

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