Future reference:
You know the pillow stuffing they sell in the fabric department? Take handfuls of that. Put them in the overflow one by one and slowly fill it up. The water will still flow through, but you are in effect raising the floor. The fish will stay above (he won't let you trap him under it). By the time you get enough in there that the stuffing is 3 or 4 inches from the top he'll be easy to catch and won't have to go on the waterslide. Then just remove the stuffing.