Most seahorses except dwarfs need rotifers for a couple of weeks.
You should put them a small tank. I use a five galon tank because in a large tank you can't keep the food near the horses. As they grow you can put them in a larger tank. You need an airstone but not too much circulation. They are horrible swimmers and a powerhead, besides chopping them up will just push them into a corner. You also need hitching posts. I used pieces of thread with a float at the top and a small weight on the bottom. If you don't provide hitching posts, they will hang on to each other and try to swim in different directions. For the newborns I use a piece of plastic window screen. They fit in the holes and hang on as the food comes to you.
You need rotifers in the tank all the time and when they grow a little, you need baby shrimp all the time. Seahorses are not like other fish. They do not have a real stomach, just a short tube and they can't store any food.
Research the type of seahorse you want to breed to see what it neds as a first food.