Thanks for the replies!
Well, my standpipes are threaded, and removeable. That will be my last resort though. I'll give the fish net a few days and just check it often. Right now the net is new and he's nowhere to be seen. Must have gone to the bottom of the chamber. Of course, it's 24 inches tall, and I can't see all the way in there.
The overflow holds 15 gallons, there's a lot of flow, and food goes by, so he'll be fine in there for a while.