Thanks, guys. I might try the media bag. (I'll probably have to sew a sleeve or something out of one.) It's a weird problem, I agree.
chappy6107: They might be after some buildup, but frankly, my shrimp aren't very bright and are probably just randomly making their way in because it doesn't happen often.
OMG Aquatics: Unfortunately, the tank is in my bedroom and the splashing would drive me nuts.
Shrimple minded: The flow is kinda weak, but definitely out and not in. Also, all the stuck ones were headfirst in the spraybar holes, like they were trying to get in on purpose. Shrimplets get blasted away, and it's only the juveniles getting in.