Ha, such an old thread, but well-timed on the resurrection. I came here looking for advice on excluding livestock not from a Skim350, but from my floating skimmer that's on the glass intake tube (jardli or whatever). I've lost otos and cories. I find shrimp living happily in the bottom of the canister filter. But usually it's the snails. Today, a snail got lodged in the intake tube, that lowered the flow, which enticed a shrimp to follow it in to dine on the dead snail. Also slowed down the CO2 reactor, which started making noise as it filled up with gas. Happens all the time. I even put a little plastic cap on top of the skimmer strainer (like you'd find on a little takeout condiment cup), but I guess the snails just push it out of the way on theor way up and over.
I was thinking of shoving some filter foam down into the top of it, but I feel like it'll reduce the skimming abilities. Maybe a plastic mesh is worth trying. This has annoyed me for years.