If you are only keeping the seahorses in the tank as you say, with no corals or live plant life, then all you need is ambient lighting or a light that sufficiently allows you to view your seahorses in a pleasing manner.
I have a spiral 13w lamp over each seahorse tank and if you get the cool white instead of warm white they work well.
I'd like to find "daylight" spirals but so far haven't come across any where I live.