I like your idea of transforming your 55-gallon aquarium into a modified reef tank for seahorses. Seahorses often thrive in the right type of reef system, which provides them with pristine water quality, plenty of roam to roam, assorted ‘pods to graze on between meals, and a colorful, natural setting that makes them feel right at home.
As you know, it is very important to assure that the corals you select are seahorse-safe before you proceed. Soft corals and short polyped stony (SPS) in general are fine, so most of the corals you currently have in your 55-gallon aquarium will work well with seahorses. Cup coral, mushrooms, green button polyps, pulsing Xenia, seafans, and zooanthids are all excellent additions to a seahorse reef. (Just be sure to observe the precautions discussed below when handling your zoos.)
To eliminate any concerns about toxins corals might release, just get yourself a good grade of activated carbon that’s low in ash and free of phosphates, add a couple of ounces of it to your filter, and replace the used carbon with fresh, clean activated carbon once a week. (If you don’t replace the activated carbon regularly, it can leach back any undesirable substances it has removed into the aquarium after it has reached its capacity.) Carbon is activated two ways, either with steam or with phosphoric acid. The type of carbon that is activated with phosphoric acid contains phosphates, which can be leached back into the aquarium water and promote the growth of nuisance algae. So you will want to avoid that type of of activated carbon. The carton or box that the activated carbon comes in should be clearly labeled and state specifically that it is "steam activated" or "phosphate free" or something to that effect if it’s a suitable brand for your aquarium.
Soft corals have very little stinging ability and generally make good choices for a modified mini reef that will include seahorses (Delbeek, Oct. 2001). This includes most mushroom anemones (corallimorpharians). 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 (Delbeek, Oct. 2001).".
Hippocampus also does very well with zooanthids and colonial polyps in general. But the hobbyist must be sure to observe a couple of precautions when handling the zoanthids and placing them in the aquarium.
First and foremost, many of the commonly available Zooanthus (button polyps) and Palythoa (sea mats) species contain a very toxic substance in their mucous coat known as palytoxin, which is one of the most poisonous marine toxins ever discovered (Fatherree, 2004). Palytoxin can affect the heart, muscles, and nerves, resulting in paralysis or possibly even death, and many hobbyists have reported numbness, nausea and/or hallucinations after merely touching these corals (Fatherree, 2004). When you handle zoanthids and palythoans, you cannot help picking up some of their protective slime on your fingers, and so much as rubbing your eye, picking your nose, or a small cut on your finger can be enough to land you in the hospital. When handling Zooanthus or Palythoa species, it’s very important to wear disposable latex gloves, avoid touching your mouth or eyes, and carefully dispose of the gloves immediately afterwards (Fatherree, 2004).
Secondly, zoanthids and other soft corals such as mushrooms may wage border battles if you place them in close proximity to each other (and the zoanthids almost always lose out to the mushrooms in these skirmishes). So be sure to allow adequate space between the colonies. Some rapidly growing Zooanthus colonies can be aggressive to soft and stony corals alike as they rapidly spread over the rockwork, but in general they are quite peaceful, and you can always slow down their rate of growth by reducing the nutrient loading in the aquarium.
Other low light corals that should be suitable for a seahorse reef 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. (Delbeek, Nov. 2001).. However, supplemental feedings of zooplankton may be required to maintain these corals in good health.
The hard or stony corals fall into two categories depending on the size of their polyps. The small polyped stony (SPS) corals have tiny polyps that extend out of minute openings in the stony skeleton, and 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 freely at your discretion (Delbeek, Oct. 2001).
The large polyped stony (LPS) corals, however, must be regarded with a little more caution. These include genera such as Catalaphyllia, Cynarina, Euphyllia and Trachyphyllia that have large fleshy polyps which often have tentacles equipped with powerful stinging cells. The Euphyllia and Catalaphyllia have the most powerful nematocysts among the LPS corals, and can deliver stings that are stronger than most anemones (Delbeek, Oct. 2001).
Unfortunately, many of the corals on your wish list that you are thinking of adding to your 55-gallon aquarium are LPS corals with potent nematocysts. This includes the frogspawn, hammer coral, and trumpet coral (Euphyllia spp.), all of which can deliver nasty stings, which is why they are sometimes known as torch corals. The Tubastrea sun coral you are also interested in is another LPS coral, but is less likely to prove troublesome.
To play it safe, LPS corals are best excluded from the seahorse reef. You absolutely must avoid the Hydrocorals at all costs, such as Fire coral (Millepora spp.) and Lace or Ember corals (Stylaster spp.), which produce fiery stings that can even injure the aquarist.
If there is an LPS coral that you simply must have — one that your tank cannot do without — then you may proceed with the utmost caution as long as you observed all the necessary precautions and choose your seahorses carefully. For example, Charles Delbeek for one has kept tank-raised seahorses in a seagrass tank with elegans coral (Catalaphyllia) and he reports that there was never any problems with the ponies coming in contact with the elegans (Delbeek, October 2001). Seahorses will quickly learn to avoid them, but it is a good idea to monitor the seahorses closely in a tank with LPS corals at first in order to assure that there are no strong currents that could sweep the seahorses against the corals.
Specimens such as elegans coral, and LPS corals in general, should be included in a seahorse reef only after careful planning and consideration is given to where they are placed and where the water returns are positioned, in order to minimize the chances that the seahorses might come in contact with them. Any seahorses that are housed with such corals should be reef specialists, such as H. barbouri and H. comes, that know their way around stony corals and instinctively know to keep their distance from the ones with powerful stings. With good planning, some LPS corals will work out just fine, but you must observe the seahorses very closely and be prepared to relocate either the coral or the seahorses if need be.
Even then, you must limit yourself to one or at most two of the LPS corals, rather than a whole tankful of them. The Prickly seahorse (Hippocampus barbouri) is very well adapted for a reef system with hard corals. In fact, H. barbouri is said to be impervious to the stings of stony corals (Wallis, 2004), so it is the perfect choice for a reef system that houses one or more LPS corals.
There have been a couple of other threads on this message board that discussed seahorse-safe corals, and you should check them out as well before you make any further plans for your 55-gallon aquarium. For instance, the following thread titled "Scared!" includes a list of seahorse-safe carols and lots of tips for setting up a suitable reef tank for seahorses:
Click here: Seahorse.com – Seahorse, Sea Life, Marine Life, Aquafarm Sales, Feeds and Accessories – Re:scared!
And the following discussion titled "what types of corals do I use?" has a good discussion about lighting options for a seahorse reef:
Click here: Seahorse.com – Seahorse, Sea Life, Marine Life, Aquafarm Sales, Feeds and Accessories – Re:what types of corals do I use?
My best advice would be to stick to soft corals and SPS corals, sir. If you feel you must try an LPS coral, plan accordingly, follow all the precautions we been discussing, try to limit yourself to H. barbouri seahorses.
Best of luck modifying your reef tank for seahorses, Christopher! Please let us know if you have any other questions in that regard.