Doing your homework ahead of time and reading up on seahorses and asking a lot of questions before you take the plunge is definitely the right approach! Maintaining optimum water quality is half the battle with seahorses, so given your background as a reefkeeper, I think you’ll do very well with these amazing aquatic equines.
Seahorses do not have any special lighting requirements other than the fact that most species prefer low to moderate light levels rather than bright light, but metal halides are often used to display seahorses at public aquaria and zoos. For example, the 2002 Seahorse Husbandry Manual indicates that metal halide lighting is often the preferred choice for displaying Hippocampus erectus at such facilities, and this applies to other species as well. Brian Zimmerman and Heather Hall maintain a very successful breeding program for H. capensis at the London Zoo, with the main tank being illuminated for 11 hours a day by 2 HQI metal halides (150 watts each, 10,000 Kelvin). Karen Brittain also kept H. fisheri at Waikiki Aquarium under metal halide lighting with a natural photoperiod. Likewise, Jorge Gomezjurado reports that he has kept adult H. ingens and H. reidi on display at the National Aquarium in Baltimore under Metal Halides Lamps (175W 6500K) that provided a photoperiod of 12:12 L:D without any problems.
So your metal halides would be considered overkill by most seahorse keepers, and not my first choice, but the pros often use them and I know many reef keepers who keep seahorses in their systems under metal halides. Often the reefers will keep the coral and inverts that require strong light at one end of the tank, where the metal halides are concentrated, and keep the other of the tank shaded to accommodate the seahorses, reserved for coral that don’t need high-intensity lights. Basically, you can’t go wrong with seahorses when it comes to lighting as long as you provide some dimly lit areas they can retreat to when they would like to get away from the light and some brightly lit areas they can move to when then want to bask in the light. If you’re using metal halides, you can provide shaded areas by positioning sections of aluminum foil atop your aquarium that are the right size and shape to cast shadows where you want them below.
As Leslie mentioned, my primary concern when using metal halides on a seahorse tank is the water temperature rather than the brightness of the light. Metal halides throw off a lot of heat and most of the tropical seahorses do best at temperatures of around 73-75°F; avoiding temperature spikes above 80°F is very important. I ordinarily recommend Mustangs (Hippocampus erectus) as the most suitable species for aquarists who are new to seahorses, but if your metal halides consistently keep your 45-gallon hex tank at 78-80°F, you lay want to consider trying a species that likes it a little warmer instead, such as captive-bred-and-raised H. barbouri or H. kuda. Or you can try raising your metal halides a little higher above the aquarium and using a small fan to blow across the surface of the water and cool the tank a few degrees via evaporative cooling, and stick with the Mustangs..
Personally, I like to provide my seahorses with a natural day/night period that includes twilight periods at "sunrise" and "sunset." To accomplish this, I like the power compact (PC) light fixtures that include two tubes — one actinic and one daylight fluorescent — with dual ballasts so that each ballast can be placed on a separate automatic timer. I like to have the bluish actinic come on before the daylight tubes and stay on after the daylights go off, thereby providing a simulated dusk and dawn. This is important for seahorses since they conduct most of their courting and breeding in the early morning hours under twilight conditions. It’s a neat effect and fish and invertebrates can then anticipate "lights out" rather than being plunged into total darkness at night or suddenly thrust into bright light in the morning. I also adjust the timers to lengthen or shorten the daylight periods in accordance with the changing seasons. (All my thanks to Jennifer Myerscough for recommending this lighting system to me – it’s working out splendidly!) I find that maintaining a natural cycle this way aids reproduction.
In short, I find PC lighting to be a good compromise for a seahorse system. Power compacts provide plenty of light for macroalgae or the seahorse-safe soft corals in a modified reef system without being too bright, and the dual ballast system allows for a natural day/night rhythm that changes with the seasons. The resulting dusk and dawn facilitate courtship and help the seahorses maintain a natural reproductive cycle.
So those of some of the things to keep in mind when selecting the lighting for your seahorses, Jerry. For all extents and purposes, you really can’t go wrong no matter what bulbs you chose as long as you provide both shaded areas where your seahorses can escape from light altogether and well-lit areas where they can bathe in bright light as they please. You will find your seahorses will move into and out of the light often, seeking the comfort level that suits them at the moment.
Whether or not you’re 45-gallon hex tank can do double duty as a frag tank and a seahorse tank will depend primarily on the stinging ability of the coral fragments. Most soft corals and SPS have relatively weak stings and are suitable for seahorses. Here are some additional guidelines that may give you a better idea as to which corals are compatible and which types you should avoid in your seahorse tank:
Setting Up a Reef Tank for Seahorses.
Seahorses typically thrive in the right type of reef system, which provides them with pristine water quality, plenty of roam to roam, and a colorful, natural setting that makes them feel right at home. The multicolored background will keep them looking their best and brightest, and nothing makes a more breathtaking exhibit than brilliant yellow and orange seahorses lazily gliding amidst the lovely corals, polyps and gorgonia in a well-established minireef, much like the butterflies adorning a beautiful flower garden.
But the hobbyist who wants to keep seahorses in a reef system must be willing to make some concessions to accommodate their special needs. For example, the reef keeper must be willing to limit himself to corals and invertebrates that meet the following criteria:
1) Avoid any stinging animals with powerful nematocysts. This means fire corals, anemones, and any corals with polyps that feel sticky to the touch must be excluded. When a seahorse brushes up against them or attempts to perch on them, the nematocysts or stinging cells of these animals can penetrate the seahorse’s skin and damage its integument. Needless to say, this causes pain and discomfort and can leave the seahorse vulnerable to secondary bacterial and fungal infections, which may take hold at the site of injury. Beware of large polyped stony (LPS) corals in particular. These include genera such as Catalaphyllia, Cynarina, Euphyllia and Trachyphyllia that have large fleshy polyps which often have tentacles equipped with powerful stinging cells.
2) The corals must be able to thrive with low to moderate light levels and low to moderate water movement or current. Corals that require overly strong water currents are unsuitable for tanks with seahorses because the seahorses are feeble swimmers and often cannot cope with powerful currents unless there are slack water areas the can retreat to when needed.
3) The corals must be able to withstand being used as hitching posts by the seahorses from time to time; that is, they cannot be so delicate that having a seahorse’s grasping tail anchored around them could cause them any harm. For instance, soft corals may retract their polyps when a seahorse perches on them. This can be harmful to their health if it becomes a chronic problem, because the corals rely on their polyps to absorb light and convert it to energy via photosynthesis. Be sure to watch any soft corals and make sure they are not closed up for extended periods. Normally, they adjust to the seahorses’ presence and unwelcome attention after a while, and remain contracted only briefly after each contact. After repeated exposures to grasping tails, each such incident elicits a weaker response, so they tend to extend their polyps sooner and sooner after being disturbed.
4) Avoid Tridacna clams and similar bivalve mollusks. Sooner or later a seahorse will perch on them with its tail between the valves and the clam’s powerful adductor muscle will clamp down on it like a vise. At best this will be a very stressful experience for the unfortunate seahorse; it can be the devil’s own business trying to persuade the stubborn mollusk to release its struggling victim! At worst, it can result in serious injury or permanent damage to the seahorses tail.
5) Beware of unwanted hitchhikers that may have come in on your live rock unbeknownst to you and which can harm seahorses, such as fireworms, mantis shrimp, or Aptasia rock anemones. When setting up a reef system for seahorses, it’s a wise precaution to pre-treat your live rock with a hypersaline drip and/or a dose of fenbendazole to eliminate such pests beforehand because they can be very difficult to remove or eradicate once they make themselves at home in your aquarium.
6) Small powerheads can be used to create and direct water currents wherever needed — just be sure to screen off the intake for the sake of your seahorses. Seahorses tolerate moderate currents very well providing there are sheltered spots and slack water areas they can retreat to when desired.
Soft corals have very little stinging ability and generally make good choices for a modified mini reef that will include seahorses. This includes most zoanthids and mushroom anemones (corallimorpharians) in general. However, as Charles Delbeek cautions, "One notable exception is the elephant ear mushroom anemone (Amplexidiscus fenestrafer). This animal is an active feeder on small fish and will envelope them whole with its mantle then slowly digest them by extruding its digestive filaments into the space created. No small fish are safe with these animals in the tank.". Some of the soft corals that generally do well with seahorses in a low/moderate light, low/moderate flow reef tank with PC lighting are listed below:
Finger Leather Coral (Lobophyton sp.)
Flower Tree Coral – Red / Orange, (Scleronephthya spp.)
aka: Scleronephthya Strawberry Coral, or Pink or Orange Cauliflower Coral
Christmas Tree Coral (Sphaerella spp.)
aka: the Medusa Coral, Snake Locks Coral, or French Tickler
Cauliflower Colt Coral (Cladiella sp.)
aka: Colt Coral, Soft Finger Leather Coral, Seaman’s Hands or Blushing Coral.
Toadstool Mushroom Leather Coral (Sarcophyton sp.)
aka: Sarcophyton Coral, Mushroom, Leather, or Trough Corals.
Bullseye Mushroom Coral (Rhodactis inchoata)
aka: Tonga Blue Mushroom, Small Elephant Ear Mushroom (rarely)
Clove Polyps (Clavularia sp.)
Stick Polyp (Parazoanthus swiftii)
Green Daisy Polyps (Clavularia sp.), Indonesia
Orange & Green Colony Button Polyps (Zooanthus sp.), Fiji
Red Ricordea (Ricordea sp.), Indonesia, occasionally Solomon Islands
Lavender Hairy Mushroom (Actinodiscus sp.), Tonga
Pimpled Mushroom (Discosoma sp.), Indonesia
Other low light corals that should be suitable include genera such as Cynarina, Scolymia and Trachyphyllia, as well as non-photosynthetic gorgonians such as Subergorgia and Didogorgia, and perhaps wire corals such as Cirripathes spp.. However, supplemental feedings of zooplankton may be required to maintain these corals in good health.
The small polyped stony (SPS) corals generally have weak stings that should not pose a threat to seahorses. Depending on conditions in the tank, SPS corals such as Acropora, Montipora, Pocillipora, Porities, Seriatopora and Stylophora can be tried at your discretion.
Best of luck with your 45-gallon hex tank, Jerry!