Honestly, it depends on the breed. Some of the dwarves (ie. Hippocampus denise, Hippocampus colemani, etc.) can live in a darned fishbowl if you worked it right. For specific needs of each species (including pipefish etc.) follow the links below :

http://www.syngnathid.org/
http://www.seahorse.org/
http://www.oceanrider.com/
http://forums.aquariumhobbyist.com/forum.php?catid=51

This article is a good one an one that I used to base my tank on (1/2 way down - "The Seahorse Tank):

http://www.seahorse.org/library/arti...sAndInfo.shtml

Dave