First, navigate the poison towards the bottom by digging a path through the dirt. The poison will hit the rock and most likely end up on both sides of the rock, clearing up the algae on the right side. Then, dig towards the narrow path to clear up the algae with the remaining poison. Then drain the remainder of the poison after that to the right side of the screen. Then dig through the dirt under the water at the top of the screen. Finally, make a path for Swampy's shower pipe and remove the dirt blocking the water at the top of the screen. Some water may get to the duck on the other side of Swampy's shower, but enough should get to his pipe.