I am going to agree with javajoe, in that tank raised horses will fare much better, but you will have to feed them often. I have a 30 gallon species specific seahorse tank, with live sand, live rock, and a very dense population of Caulerpa. The caulerpa harbors copeopods and small amphipods, both of which the seahorse will spend all day hunting. The problem that i see is that your three efish that you have will be competing with the horses for these foods. Even if you feed the horses three times a day with mysid shrimp like i do they will still spend the rest of their lives hunting and eating, and if they have to compete with three other fish with similar tastes, they are not going to have a chance. Stay away from wild caught specimens. THey are nothign but trouble, and you will have a hard time getting them to take to frozen food. If you do purchase a wild horse, with the three fish you have, it will undoubtedly starve to death as it will only feed on live food. I have trained two wild horses to accept frozen foods, but believe me, it was a pain and took about 3 months. Wild horses are also usually separated from their mate in the wild, and suffer from this because seahorses form pair bonds and mate with only that individual for the rest of their life. Therefore, if you get a wild caught specimen he or she was most likely removed from his partner and will not last in captivity. Wild horses are also much more succeptible to bacterial infections, whcih can include, disintegration of their snout, making them unable to capture food, gas bubble infections in males who trap air bubbles in their pouches, and numerous other bacterial infections that will appear as spots on the tail, or body. If you are serious about pursuing seahorses in yor tank, knowledge is the key. Read anythign and everythign you can about these guys. <a href="http://www.oceanrider.com," target="_blank">www.oceanrider.com,</a> is probably the best outlet for captive raised horses, and i hav enothign but praise for them, four of my 5 horses came from them, and i have had them for about three years now. They have some great info on their site. Another excellent site is <a href="http://www.seahorses.org," target="_blank">www.seahorses.org,</a> and a great book is Seahorses: an identification guide to the world's species and their conservation by amanda vincet and Sara lourie. These guys are amazing creatures, and are fascinating to watch, but do require a little more time and care than regular fish species!
good luck
jon