There's actually a seahorse site dedicated to only sea horses. There's a lot of very knowledgeable people on that site on keeping sea horses. I kept them before in quite an interesting set up. I had a 350 gallon pond that had a bat ray in it which jumped out and died so I decided to throw in some sea horses. The system as a whole was in excess of 500 gallons, but I put 5 hippocampus reidi (if I remember correctly, basically, the Brazilian sea horses) in it, dumped in a shipment of 1000 mysis shrimp into the refugium along with a bunch of grass shrimp from someone, and let them roam freely. There was a lot of flow in the pond area, but when it came down to it, they were pretty happy as there was also plenty of room to swim in low current areas, plenty of rock to search around in, and a mat of macroalgae the size of a small child to hunt in. They also were trained fairly easily to eat frozen mysis as well. Anyway, I think the name of the web site is seahorse.org but I'm not sure. Some of the members here are regulars on it.