Yes, sir, upgrading your lighting and concentrating on SPS corals is one option for a seahorse reef tank. Metal halides would work just fine for such a system providing they don’t generate too much heat and that you provide plenty of shady retreats as well as the brightly lit zones with high-intensity lighting.
Or you could stick with soft corals that require less intense lighting, which would allow you to save some money on your lighting system and to provide your seahorses with a natural photoperiod, including a simulated "dusk" and "dawn" to facilitate courtship and breeding.
Or you could do without the corals altogether and establish a lush bed of macroalgae to simulate a seagrass habitat instead of creating a reef biotype. For example, I prefer decorative marine plants or macroalgae in a variety of shapes and colors and color — reds, browns, golds, and yellows in addition to green varieties, some tall and feathery, some short and bushy — to provide natural hitching posts and shelter for my seahorses. I like to start with a mixture of red and gold Gracilaria (Ogo) and artfully arrange them around a lush bed of assorted bright green Caulerpa. Any of the plumed (feathery) or long-bladed Caulerpa would be ideal for this, such as Caulerpa sertularioides, C. mexicana, C. ashmedii, C. serrulata or C. prolifera. The result is a colorful macroalgae garden with a very nice contrast of colors (reds, yellows, greens, and brown) and interesting shapes. A tank heavily planted with macros such as these is a lovely sight and mimics the seahorses’ natural seagrass habitat well.
As an added benefit, the macroalgae act as an excellent form of natural filtration, reducing the available levels of phosphates and nitrites/nitrates. Be sure to prune and trim back the fast-growing Caulerpa regularly; when you remove the clippings, you’re exporting phosphates, nitrates and other nutrients from the tank, thereby helping to maintain good water quality, and pruning the runners helps keep it from going sexual.
When pruning or trimming back macroalgae, take care not to actually cut it. Remember, you’re not pruning hedges or trimming trees — the idea is to carefully pull up and remove continuous, unbroken fronds. Simply thin out the colony of excess strands, gently plucking up convenient fronds that can be readily removed intact. A little breakage is fine, but cutting or breaking too many strands will result in leaching undesirable substances into the aquarium water as the Caulerpa’s lifeblood drains away. Too much cutting or breaking can thus sap the colony’s strength and cause die offs or trigger the dreaded vegetative events that judicious pruning otherwise prevents.
If you have any concerns about your ability to maintain and control of Caulerpa properly, just use a different forms of macroalgae that grows less rapidly instead and you can get the same sort of benefits at relatively little risk. In that case, some of the other macroalge you may wish to consider are Gracilaria, Ulva, Chaetomorpha, and Chlorodesmis. Hawaiian Ogo (Gracilaria sp.) are bushy red-to-brown macros that do well under low light levels. Sea Lettuce (Ulva sp.) are deep green sheets of algae that do best under a little stronger lighting. Maiden’s Hair (Chlorodesmis sp.) are bright green tufts or clumps of very fine-bladed algal mats to grow attached to small rocks. All of these types of macroalgae are much less prolific and slower growing than Caulerpa. However, like all macroalgae, they should still be harvested periodically in order to export the excess nutrients they have consumed.
Aside from red and brown Gracilaria and the bright green Ulva and Maiden’s Hair, some seahorse keepers also like the Chaetomorpha turf algae. It can best be described as looking like the clumps of the colorful plastic grass we use to fill Easter baskets. It is popular because it is slow growing and doesn’t require the kind of pruning that Caulerpa needs, and because it it comes loaded with microfauna: miniature feather dusters, copepods and amphipods, tiny snails and micro stars. Sounds like another interesting marine plant that can add some extra variety to a lush bed of macroalgae.
Best wishes with all your fishes (and invertebrates), Christopher!