Rats! I’m very disappointed to hear that you aren’t able to use the metal halides and canopy that came with your aquarium upgrade; if you have to replace the lighting system, that makes the upgrade that much less of a bargain. Still a very good deal, but I know the gargantuan task you undertook to get the new aquarium set up your house, replace your old 55-gallon tank, and transfer all the corals, seahorses, and other specimens to the new system with the least possible stress, and I badly wanted everything to go smoothly for you. You did an excellent job with all of that and pulled it off nicely, only to discover that the metal halides on this tank are a bit too overpowering for the seahorses. That’s a darn shame…
I know many hobbyists who do use metal halides on seahorse tanks quite successfully, but there are a few tricks to it. For instance, you have to avoid overheating and you have to provide the seahorses with well-shaded areas they can retreat to when they would like to get out of the bright light. Often seahorse keepers accomplish this by confining the metal halides to one end of the tank, while keeping the other end of the tank relatively dimly illuminated, or using pendants for the MH to use them as spotlights to sort of highlight certain areas of the tank or corals that require high intensity lighting, leaving the rest of the tank well-shaded. And often the metal halides are suspended above the aquarium, rather than mounted in a canopy. That way, you have a little better control over both the brightness and the heat generated by the lamps simply by raising or lowering the suspended lights as need be.
Best of luck tweaking your lighting system and working out the perfect arrangement that works great for both your seahorses and live corals, Carrie!