If you're talking about the overflow box, like what is on a wet-dry set up like mine, I'm not sure how to get it out except to shut down the filter and pull out the U-tube. Then you could remove the box to put him back in the tank. Of course, you'd have to set up the U-tube again. I do know that I had that problem with small fish going thru those slots and found a remedy for that. I took a strip of plastic canvas (found at any craft store) and wrapped it around the top edge (where the slots are) of the overflow box, held in place with a rubber band. I've had no problem since.