The link in my signature has links at the bottom of my page that provide you with the basics you need to know for seahorse keeping.
Basically, for BEST chances of success, a species only tank that has temperature controlled to the lower 70's with excellent tank husbandry.
Seahorses number one killer is bacterial like vibriosis, and the next biggest killer is acquired pathogens from other tank mates that have been or are in the tank presently, that the seahorse hasn't been reared with to acquire some immunity for their effects.
By keeping the water cooler than seahorses may normally be found, you limit greatly the chances of deadly bacterial plagues.
MOST seahorses are very selective in the food they eat, with some eating almost everything but most picking and choosing which pieces they eat. The remainder is a major cause of water quality issues that aid bacterial infestations. Some can be trained to eat from a feeding dish, while others may only pick out of the water column.
There are claims that seahorses don't like much flow but in fact are comfortable as long as they don't get blown against objects by a blast of water, and having areas with hitching in low, and medium flow so they can choose their place at any given time. Sometimes some seahorses actually "play" in a high water flow.
Another major thing to succeed in the hobby is to buy only TRUE captive bred seahorses. While many places claim to sell captive bred, most are actually tank raised or net pen raised, with a few wild caught ones showing up from time to time. These are all grown in water that is insufficiently treated and filtered to remove the pathogens from the ocean water used and as such, need to be put through a 9 week protocol with 3 separate drugs to give a better chance of survival.
The best recommended source in the US for seahorses is seahorsesource.com where you not only can get seahorses and related supplies and medicines, but the best advice on seahorse keeping available from any breeder.
