What overflow do you have on that tank? HOB
or a drilled tank? how is he able to fit thru there to get down to the sump itself?
Ive got neons living in my overflows and not once has one made it to the filter. Not on my 52 with a CPR hob
overflow I get fish mainly my black kuhlli's that get into the overflow and cant make it past that.
I dont think water parameters have anything to do with this because its only the yo-yo you are having issues with. QT him for atleast 2 weeks and observe his behavior and health.