since you dont have spoiler in the heading, i will make invisable text so that you can see what you need to do.
To see the text right click and highlight everything from where i say "start" to "finish" and you will be able to read what i have put, ok.
SPOILER
START
to get the puzzle solved and remove the algea you need to get the algea eating fish from the pond to the ocean. it will take more then one, about 10 or so depending on what game speed your running at, just keep picking up your villagers after they drop the fish in the ocean, and put them back in the pond till you get the messages saying you solved the puzzle. once you get that, you will not need anymore of the algea eating fish.
FINISH