Don't just go buy on a whim. Read up on them first if you want some in the future- but you don't know what species the seahorse is, where it came from, etc. Damsels and clowns are also really bad tankmates.
If your interested later on trying them, check out seahorse.org. I know I keep posting this forum address, but it's the best place to start with these fish.
Seahorses are harder than other fish at times, in certain conditions, but are hardy when conditions are right- and you know what your doing. One of the biggest downfalls is the many problems that can hit them, and diseases are often a huge deal, rather than something as simple as ich in a regular tank.
HTH