"Hard" is a relative thing. What is hard for me may be easy for you.
Seahorses new special care compared to a normal salt water tank. Water conditions need to be excellent, in spite of the fact most seahorses are "messy" eaters with a lot of food usually left uneaten. This means many times, a special regime to clean up the uneaten food before it changes into ammonia etc......
Because of their propensity to succumb to bacterial infections, we need to keep them at cooler temperatures than they would be found in under natural habitat conditions.
Like any aspect of the salt water hobby, there are differing opinions but if you click the link in the second post above, you can see my thoughts on keeping them, plus there are some links at the bottom for further, more knowledgeable information.
Use the google or other search engine to round out your information, especially the articles in the two main seahorse forums.