Most tank raised seahorses now are raised feeding on frozen mysis and brine.
I still raise Artemia Salina (brine shrimp) and Artemia Nyos ("Sea Monkeys") to feed them. 1/8th teaspoon brine eggs, 1/8th teaspoon DT's or other phytoplankton in 12 ounces of your tank water will hatch them in less than 24 hours. You can introduce the naupali or wait untill they get bigger. 3 day old naupali can usually swim against the current in a seahorse tank, since they also like slower water movement. Ive even introduced full grown sea monkeys into the tank, 1/2 inch to 3/4 inch long. They went right into the live rock, and have since reproduced on their own.
Anyway, Im rambling. Moral of the story, GET TANK RAISED SEAHORSES! There are too few left in nature to be harvesting them still, and those are the ones that dont do well in a tank. Its easier than you think, if you have an established tank. ALso, Mandarins are an excellent tank mate for seahorses, as are most small (<1 inch) gobies.
HTH!