Due dates appear next to each of their assignments on your Fishtank Student homepage. Assignments also include labels indicating whether they are due today or are past due (screenshot below):
If you have rostered your class using Google Classroom, your students can receive notifications when new Fishtank Student assignments are posted and reminders when they are due through their Google Classroom portal. To do this, select "Post to Google Classroom" under Additional Settings when editing an assignment (screenshot below): 