I can't say much about Seahorses, but Doryrhampus species Pipefish (D. excisus, D. melanopleura, D. janssi,...) would need to be very sick and weakened to be sucked into a pump. Mine (I have several pairs in my reef tanks) like to hang out scarily close to my Jebao WP 40 and 60 pumps and the pipefish would at least in part fit through the intake slots. These pipefish have no trouble going against flows that make regal angels or other larger fish struggle.