If you search the topic you'll find this has been discussed to death, but I'll sum it up for ya.
Problems include:
size of refugium and height to give seahorse swimming room
flow through the refugium
seahorses will decimate the pod populations in a matter of days so it will no longer be a refugium
the temp of the refugium is usually not suitable for seahorses, they need temps close to 74F
Seahorses are dirty creatures so if it's for your reef with pristine water conditions, forget it, or really increase your filtration.
Hard to view in the sump, kinda a waste.
It's been done. But to do it well is a challenge.
