Seahorses are very difficult to keep. They need a mature tank with lots of pods for them to eat. They also need some form if hitching posts to hang on as they aren't very strong swimmers. They can't be kept in a reef and need a dedicated tank of their own. They need nice gentle flow. They can't be kept with most corals as they will be stung.
They mostly feed off live pods which is why a mature tank is needed. Preferably with macro algae. You can also feed them live brine and if you're lucky they'll eat frozen foods.
Tell us more about your tank. Seahorses are always interesting.