I agree with Sissy and Nova regarding the live rock. With the exception of dwarf seahorses (Hippocampus zosterae), my preferred setup for keeping the larger Hippocampus species is a "seahorse-only-with-live-rock" tank.
The one indispensable part of a SHOWLR system is the foundation of live rock. The live rock is the living, breathing, heart and soul of the system, which provides the bulk of the biological filtration as well as some denitrification ability and shelter and habitat for countless critters and microfauna. The porous interior of the rock supports large populations of the beneficial oxygen-loving Nitrosomonas and Nitrobacter bacteria that breakdown deadly ammonia and nitrite into less toxic substances (primarily nitrate). Deeper inside the live rock, where oxygen levels are nil, anaerobic denitrifying bacteria take hold and complete the nitrogen cycle, breaking down nitrate into harmless nitrogen. This helps keep the nitrate levels in the seahorse tank low. As a result, live rock is superior to most other forms of biofiltration, which lack this final anaerobic step and cannot carry out denitrification. This makes live rock doubly good at maintaining optimum water quality.
Equally important, the rockwork provides cover for the seahorses. By this, I mean the rock allows the seahorses to hide and conceal themselves completely whenever they feel the need. Seahorses are shy, secretive creatures that rely on camouflage as their primary means of protection, and if they feel too exposed and vulnerable, it can be stressful for them.
As much as 1-2 pounds of live rock per gallon is recommended if the live rock will be the primary means of biological filtration in the aquarium. That amount of live rock will provide adequate levels of both nitrification and denitrification for the tank. That amount of live rock will provide adequate levels of both nitrification and denitrification for the tank. (However, if you will have an additional means of biological filtration on the aquarium, then you won’t be nearly that much live rock and you can get by with a fraction of that amount.) You can simply select the precured live rock you find most attractive at your LFS and add enough of it to create interesting rock formations that are aesthetically pleasing to your eye. Use enough rock to create some interesting caves, arches, ledges and overhangs.
Despite its beauty, natural appearance and the many benefits it provides, some hobbyists avoid live rock like the plague for fear that they may introduce harmful pests to their aquarium along with the live rock. This is a valid concern since potentially harmful hitchhikers like mantis shrimp, fireworms and large bristleworms, aggressive crabs, hydroids and Aiptasia rock anemones are very often unseen and unwanted tenants of live rock. They insinuate themselves throughout the live rock in nooks and crannies, and multitudes of these squatters may have taken up housekeeping in a good-sized piece of rock unbeknownst to the unsuspecting aquarist. They conceal themselves within the labyrinth of rock and often escape even the closest scrutiny undetected.
But with a little care this is one time when aquarists can have their cake and eat it too. There are a number of ways to take advantage of all the benefits live rock provides without risking unleashing an epidemic of tenacious rock anemones or turning Jack-the-Ripper loose in your tank reincarnated in the form of a thumb-splitting Stomatopod.
For instance, some seahorse keepers treat live rock with fenbendazole (brand name Panacur) for a period of 3-4 days to eradicate such pest before placing it in the aquarium. Fenbendazole is an anthelmintic agent used for deworming large animals such as horses (Abbott, 2003), and 1/8 teaspoon of granular fenbendazole per every 10 gallons of water will indeed kill worms, including bristleworms, as well as hydroids, anemones and certain other Cnidarians (Liisa Coit, pers. com.). It will not harm your biofilter, most macroalgae, copepods or most types of shrimps and other crustaceans, or most kinds of snails (Liisa Coit, pers. com.), so it leaves most of the desirable life on the rock intact. Be sure to check the water for ammonia and nitrite if you use fenbendazole for pest control this way, since the sudden die off of worms and Aiptasia anemones is likely to cause ammonia and/or nitrite spikes that must be controlled with water changes. However, the porous live rock will absorb some of the fenbendazole and gradually leach it back into the aquarium for many months afterwards with harmful results for live corals, gorgonia, tubeworms, Christmas tree worms, and the like, so you should never treat the live rock that will be used in a reef tank or system with live corals in this way.
By and large, bristleworms are beneficial scavengers and sand sifters unless their numbers get out of hand or they reach of size of 3 inches or more in length, so a better option for many seahorse keepers is to keep the Aiptasia and bristleworm population in check using some means of biological control. Peppermint Shrimp (Lysmata wurdemanni) love to dine on Aiptasia rock anemones and several of these attractive shrimp will do a fine job of eradicating them from the aquarium. Certain nudibranchs (Berghia sp.) also feed on Aiptasia. Likewise, small Arrow Crabs (Stenorhynchus sp.) will keep the bristleworm population at a manageable number. Any mantis shrimp or aggressive crabs that happen to slip by are generally fairly easy to trap and remove, and commercially made traps are available for that very purpose.
Treating the live rock with a hypersaline dip is a good technique for ridding it of unwanted pests. This method doesn’t kill the critters outright but merely drives them out of the rockwork so you can selectively cull through them. Another advantage of this method is that leaves all the desirable life on the rocks intact and unharmed.
To use this technique, simply place your newly arrived live rock in an inert container filled with saltwater at a specific gravity of 1.045 to 1.050 or higher for several minutes before you introduce it to the aquarium. Invertebrates cannot tolerate rapid changes in salinity, so all the mobile inverts in the rock will immediately abandon there hidey-holes and bale out of the rock like rats deserting a sinking ship. After several minutes in this extra-salty bath, the evacuation will be complete, and you can remove the now pest-free live rock and sort your catch. Cull the invertebrates left behind in dipping container, discarding the pests you don’t want while retrieving any of the refugees you might like to add to your system. Five minutes in the hypersalinity is enough to drive out all the active invertebrates such as mantis shrimp (Stomatopods), crabs, and assorted worms of every description, yet this brief period of immersion will not harm encrusting organisms or sessile life.
Another easy way to debug your live rock of unwanted pests is the Club Soda trick. To use this technique, simply remove the live rock to an empty bucket and flush out the likely looking hidey holes thoroughly with a generous amount of straight, undiluted Club Soda. The carbonation in the Club Soda means the hidden pests will be immersed in CO2, deprived of oxygen, and subjected to a drastic pH shift all at once. They’ll bail out of their hiding places in a big hurry! You can then rinse the live rock in a bucket of saltwater you have prepared in advance, to remove any lingering traces of the Club Soda, and return it to the aquarium immediately.
This method works well for surgical strikes in which you are flushing out a particular pest whose hidey hole you have already located. If you want to cleanse your live rock of unwanted hitchhikers in general, place the live rock in a bucket with just enough saltwater to cover the rock and then add a full 2-liter bottle of Club Soda (Liisa Coit, personal communication). Pour the Club Soda slowly over the surface of the rock, concentrating in particular on any cavities or crevices. This will drive out any mobile pests hiding in the rock, including crabs, mantis shrimp, pistol shrimp, and bristleworms, in a matter of moments. Afterwards the live rock is rinsed in saltwater to remove the residual Club Soda, and is ready to be returned to the aquarium. (Any desirable critters that may have been driven out of the live rock — Gammarus, pods or snails, for example — can be netted out of the delousing bucket and returned to the aquarium as well, none the worse for wear.)
So if I was you, I would avoid the hassle curing the live rock and instead select attractive pieces of precured live rock from a good LFS that caters to reef keepers, and then debug the live rock as described above before you introduce it into your seahorse tank. That way you can enjoy the benefits of the live rock while minimizing the potential for any harmful hitchhikers to find their way into the aquarium. And once your tank has been well-established and build up a suitable population of copepods and amphipods amidst the live rock, a Mandarin goby should do well in such a setup.
Best of luck with your new seahorse tank!