Fish keep getting into Overflow

[Minkota]

Have a 150g tank with a Corner Overflow, my issue is my small Chromis and Clown Fish keep getting into the Overflow. Last night BOTH of the Clown Fish got in the overflow and one ended up getting sucked down into the Wet/Dry unit and was not doing well this morning and I expect he will be gone when I get home tonight. Has anyone else had this issue? What can be done to prevent it. There are no openings other than where the return comes out.