What I've discovered with the seahorse is that it helped keep my rare babies alive longer before I upgraded my tank. It seemed to work the the adults who wouldn't quite thrive in my regular tank, as well.

Without the seahorse in the tank, I'd get a baby that would die as soon as it hit 5. With the seahorse, it'd live through part of 5 and maybe hit 6 before dying. I didn't really find it that useful though, as any adult that looked like it was fragile I sold off to fund a tank upgrade.