ghostrida3,
the slots are pretty small, so the issue won't be sucking fish and killing them, the main issue if any, is that if its off fish might get in from the front and when the pump starts, kills the fish,
my pumps, tunzes, when off vibrate every few, therefore scaring fish into entering the pump,
but I find that running interval 2, keeps one pumps doing is thing and the other a the lowest setting, instead of completely off, that way its safer
I have two and many small fish and never had any problems,
the link above is a good idea, but with high speed pumps, and an anemone that lands into the pump, something bad is going to happen,
good luck,
sam