Seahorses are saltwater fish and so lovely and charming and fantastical to watch. If you have a hankering for trying your hand at caring for these graceful creatures and keeping them as a part of your family - please just realize that this is a little bit advanced. (Well worth the effort, though.)
First of all, of course they are for your saltwater tank. You can only buy them where you will find saltwater fish - such as your local independent aquarium store.
Your small, adorable seahorse is difficult to feed at this young stage of its life. (They eat shrimp.) It's better to get one that's a little bigger so its chance of survival is better. If it's made it that far when you take him home, he's probably going to be fine if you are prepared to care for him properly. It's just recommended to not bring them home when they are so tiny because of the difficulty of feeding the tiny ones.
Be aware, also, that, although your cute little seahorse may be small when you get him, they can grow to 10 inches tall! You'll have to plan for enough room for them in your tank!
They are fascinating and enchanting creatures who deserve our best love and care and attention.