Most powerheads will have a input 'diffuser' which is basically a plastic insert that extends the area by which water is drawn into the pump. What this does is reduce the suction force by spreading it across a larger surface area. Less suction = less chance to suck something up (like a fish).
I will be hosting the Q+A at the meetings so if you find that you are not getting a fast enough response here, by all means, I can put it out there to the club at the meeting for more dialog. Flow types and methods are a great subject and I am looking to book a guest speaker sometime this year on this very topic. Fun stuff!