Seahorses are difficult to keep, like others have said, but can be kept in a reef with caution. I have had seahorses and had minor issues, but overall it as been a good experience.
Tank size is not realy important, however how TALL is your 30 gallon? They need at LEAST 21 deep inches of water-- if your tank is less than that (from top of substrate to top of water) then you really should not attempt. Also, if you are new to the hobby, i recomend that you wait a while, do tons of research. check out seahorse.org - lots of good info there, and they have forums as well.