I have had a brazillian seahorse in my reef with no issues at all. He's been in there for 2 months. He eats cocepods, and also munches on my blue leg hermits!
i know that people say not to keep a seahorse in a reef, but mine has been fine. I had a problem with my first seahorse, in that i had an agressive lawnmower blenny who attacked and killed him. Hes gone now, so my seahorse is predator free.