With keeping seahorses you need to have the right enviorment for them. First of all your tank would be better if it was tall (as mentioned above) because seahorses like the water to be deep. Also they like to have lots of things to hang onto such as plants, feather dusters and rock. I found that a very good rock for seahorse tanks is the tonga branch live rock. This rock has branches witch is perfect for seahorses to grab onto with their tails. Also these creatures are very delicate and need the right kind of care so you need to know what you are doing. Mostly seahorses love to eat coepods (like the mandrains do), they like live brine shrimp too a lot. Some people find it actually easier to raise brine shrimp for their horses. There shouldn't be any fish with the horses except mandrains and pipefish. The horses and pipefish get along well as the seahorses like to hook their tail onto the pipes and get rides on them.
They like a little current but not very much because if there is too much for them they could injure themselves by getting blown around in the current. I also wouldn't reccommend getting things like cleaner shrimp or crabs that may bother the horses witch can lead to stress.
But if you want to learn a lot more about seahorses setting up their enviorment, tank mates, and more I reccommend going to <a href="http://www.seahorse.org," target="_blank">www.seahorse.org,</a> they have a message board and evything you need to know about keeping seahorses. Hope this has been help to you and good luck!