good comment. I notice small snails in my filter socks, guessing with a single gate, the opening is large enough they just pass through (if was running 2 valves the opening in each would be narrower).
Screening the primary only?
I screened both. A little PVC, chopping the ends off of an elbow, and some bird netting:
Inserted the in the bottom of the siphon and durso elbows. This morning there was a snail stuck on the siphon screen; the restricted flow was causing it to cycle through startup mode. Unfortunately due to my canopy and the size of the weir, it's awfully difficult to get my hand in there to clean things out... but a little creative bending of our net and I was able to sweep the snail out of the way after shutting down the main pump.