Like teresa said, you can do seahorses in a reef, but you have to be uber careful planning it out. LPS corals and anemones can sting badly and kill horses, many reefsafe fish are not seahorse-safe, and invertebrates need to be monitored: some, like red-leg hermits or emerald crabs, can antagonize a seahorse, while others, specifically skunk and peppermint shrimp, will be viewed by the horses as potential food targets.
If you want to display horses in a reef environment, it may be best to start from scratch with a new tank or completely cleaned existing tank. Plan all the other living inhabitants AROUND the horses, and you will be doing well.