The dimensions of the 30 gallon Extra-High All-Glass Aquarium recommended in the training course are 24"L x 12"W x 24"H. In other words, the recommended seahorse setup is considerably taller than your 29-gallon tank that measures 30"L x 12"W x 18"H. The water volume or capacity of the two aquariums is very similar, but one of the tanks emphasizes length while the other one emphasizes height or water depth. In such a situation, the taller, deeper aquarium is always more suitable seahorses.
With regard to mating, Mustangs and Sunbursts (Hippocampus erectus) of average size should certainly be able to mate successfully in your 29-gallon aquarium that is 18 inches high. Younger, inexperienced seahorses generally need more height during the copulatory rise in order to transfer the eggs and mate successfully than older, more experienced breeders do, but the urge to reproduce is very strong in Hippocampus, and the ponies will usually improvise, adapt, and overcome in order to achieve coitus in an aquarium that is 18 inches high. There may be some spilled eggs at first, but they should eventually get it right.
However, the water depth of your seahorse tank is important not only to assure that the ponies can mate comfortably but also to protect them from depth-related health problems such as Gas Bubble Syndrome (GBS), as explained in more detail below.
This is normally what I advise home hobbyists regarding the height or water depth of their seahorse tanks, sir:
Seahorse Setups: Taller Is Better
A reasonably tall tank is very desirable for seahorses. Although they are demersal animals that orient to the substrate and attach themselves to convenient hitching posts on the bottom using their prehensile tails, they will swim and explore the different parts of the aquarium from time to time, including the upper regions of the tank The seahorse’s upright swimming style is best suited to tall aquaria.
In fact, seahorses must rise through the water column together in order to mate, and if the aquarium is not tall enough, it can interfere with breeding and mating. The actual transfer of eggs takes place while the couple is suspended in midwater or slowly descending toward the bottom — a maneuver that is every bit as tricky as it sounds. Coitus is marked by an extremely awkward, fleeting embrace, aptly described as little more than a brief belly-to-belly bumping (Vincent, 1990). As you can imagine, many difficult and delicate maneuvers are required to bring the pair into proper position for this most improbable merging. The female will attempt to insert her oviduct into the gaping aperture of the male’s inflated brood pouch. An inexperienced pair will often end up misaligned, perhaps at right angles to one another or with one of the partners too high or too low to join. This is very typical of the many false starts and abortive attempts that are ordinarily involved. The frustrated couple will separate to rest on the bottom prior to successive attempts. They may require many such rises before the proper positioning is achieved and the crucial connection is finally made.
The point is that, if the aquarium is too shallow, the seahorses may be physically unable to copulate and transfer the female’s eggs into the male’s pouch for fertilization. This is an important consideration when breeding the largest species such as Hippocampus ingens, H. abdominalis, H. erectus and H. reidi. For instance, Australian breeders report that reducing the water level to below 500 cm or about 20 inches deep can completely interrupt reproduction in Potbelly seahorses (H. abdominalis; Michael Dickson, pers. com.). Suffice it to say that Mustangs and Sunburst (Hippocampus erectus) are among the greater seahorses and they require a reasonably tall aquarium in order to mate comfortably.
More importantly, a tall aquarium can also help protect the seahorses from depth-related health problems such as bloated pouch and certain forms of Gas Bubble Syndrome (GBS). This condition is caused by the formation of gas emboli within the blood and tissues of the seahorse, which sometimes happens due to gas supersaturation of the water but is more often associated with changes in the blood chemistry of the seahorse (i.e., acidosis).
Water depth is protective because the deeper you go and the greater the water pressure becomes, the more dissolved gases the water (and one’s blood) can hold in solution. By the same token, the shallower you go and the less water pressure there is, the less dissolved gases the water can hold and the more likely gas is to come out of solution and form gas emboli (i.e., seed bubbles) in the blood and tissues. (A bottle of champagne is an extreme example of this phenomenon: as long as the bottle remains corked, it is pressurized and all of the carbon dioxide and dissolved gases it contains remain in solution, so there are no bubbles. But as soon as the bottle is uncorked and the pressure is released, the bubbles come out of solution and the champagne bottle erupts with a geyser of foam.) And that is why SCUBA divers can be afflicted with the "bends" if they come to the surface too rapidly after a deep dive — dissolved gases will come out of solution and form bubbles (gas emboli) in their bloodstream and tissues, which is very painful and can cause all sorts of dire problems, including paralysis or even death, depending on where the gas bubbles form and lodge in their bodies. The relationship between pressure and dissolved gases in a solution means that there is a definite relationship between Gas Bubble Syndrome (GBS) in seahorses and aquarium depth. To put it in a nutshell, the shallower the water depth, the more likely GBS is to occur.
As a result, shallow tanks under 20 inches in height are associated with a higher incidence of gas bubble disease in seahorses. To cite just a couple of examples, Karen Brittain, the curator at Waikiki aquarium, reported chronic gas bubble disease problems with the Hawaiian seahorse (Hippocampus fisheri) when they were maintained in shallow aquaria. However, she found that gas bubble problems greatly decreased when the H. fisheri were transferred to taller tanks.
Likewise, Mic Payne (an aquaculturist who is raising several strains of cultured seahorses at the Seahorse Sanctuary) experienced recurring problems with GBS due to maintaining a population of Hippocampus subelongatus in shallow tanks only 16-inches (40 cm) deep:
"Seahorses maintained in this system are susceptible to gas bubble disease. Specimens with bubbles around the eyes or under the epidermis of the tail are readily treated with acetazolamide (Diamox tablets 250 mg). Mix a very small amount of crushed tablet with water and inject it into several glass shrimp that are then frozen. These are then fed to the target animal at the rate of two per day for four days. Bubbles disappear on the second day (Michael Payne, pers. com.)"
The upshot is that the greater hydrostatic pressure at increased depth is known to protect seahorses against GBS, whereas the reduced hydrostatic pressure in shallow aquaria is known to be conducive to GBS. For this reason, it is important to select an aquarium at least 20 inches tall (the taller the better) for a seahorse setup. In my experience, seahorse tanks that are at least 20-24 inches tall are much less susceptible to problems with gas bubble disease that aquaria that are shorter than 20 inches. There is considerable evidence that tanks 3 feet deep or more provide excellent protection against GBS. This is because the gas emboli that cause GBS form more readily at reduced hydrostatic pressure, and will go back into solution again if the hydrostatic pressure is increased sufficiently, and obviously the deeper the aquarium the greater the hydrostatic pressure at the bottom of the tank.
However, aquaria that are 36 inches in height or taller are generally impractical for the home hobbyist. For one thing, tanks that tall are not readily available and usually have to be custom made, which often makes them prohibitively expensive. For another thing, it is difficult to work in an aquarium that deep in order to aquascape it or perform the necessary servicing and maintenance. I find that a height of 24 inches is a good compromise for most hobbyists. Tanks that size are readily available, reasonably priced, and provide at least a measure of protection against gas bubble syndrome. And I not recommend keeping seahorses for long periods in aquariums that are less than 20 inches tall.
So the height of the aquarium is a very important consideration when keeping seahorses, Brad. They need a reasonably tall aquarium in order to mate comfortably and to provide them with protection from depth-related health problems. But they also appreciate horizontal swimming space, and will be more active in an aquarium that has good bottom space as well as good height. (The females in particular tend to be more footloose and fancy free than the stallions.)
Best of luck finding the perfect tank for seahorses, sir.