I have a brazillian sea horse in our reef tank, and we love it. We had a spotted seahorse, but one of its tank mates was a lawnmower blenny. They got a long fine for several weeks, then the lawnmower blanny decided enough was enough, and attacked the seahorse repeatedly... he finally ripped part of his back off before we seperated them. I tried to nurdse the seahorse back to health, but did not work. We removed the blenny, and now have anotehr seahorse. He hooks on the the live rock, and i dont even have to feed him, as he is constantly eating pods. there is enough areas in my tank with low current so he can rest, but he also LOVES to play in the stronger current. He will swim right into it, and he knows just where to turn to get a free ride across the tank.
I will echo whatother people will say to you though, seahorses probably DO belong in a tank by themselves, and they DO require pristine conditions. I would recomend that you get tank raised seahorses, for 2 reasons. 1- helps to prevent depletion of the seahorses in the wild, and 2- they are more likely to eat frozen shirmp-- live ones almost allways have to have live food. Mine is tank raised, and will tank shrimp if i feed him.
Also, please keep in ming that they WILL affect yout BIO load, i would recomend adding only one or 2 at the most at a time. Give your tank a chance to adjust... they DO crap a lot!