Bruce, there is an impeller in there. The blades are widely spaced, and there's a chance that small fish won't get hit. If you pump 100 GSF or BH fry or eggs through there, I bet 5-10 would survive the 400 G's and slashing blades.
I will be pumping from a small gully that has GSF, BH, gambusia, and God knows what else. But I will be filtering the water through a shallow, grassy 800 foot ditch, and a large dam of limestone gravel.
