Lizzie,
Noticed your question while browsing the community. You can create a workflow with an enrollment trigger of Task Status = Completed. When creating the workflow, click Set enrollment triggers, and choose “Activity Properties” as your Filter Type.
You only have to consider if you want the same thing to happen after ANY task is marked as completed. You would probably need more logic in your enrollment trigger, such as “Task Title Contains,” in order to not trigger the workflow for every task.