Seahorse Gastroenterology
One of the peculiarities of seahorses that makes them more difficult to keep than other marine fish is their tremendously abbreviated digestive tract.  The distance between a seahorse's mouth and its anus is so short that when it eats something, there is precious little time for its gastric system to draw nutrients from the meal before it is eliminated, and, therefore, a seahorse has a very difficult time storing energy.  The best way to combat this evolutionary predicament is to supply your herd with highly nutritious foods in relatively large quantities each day.  This will allow for the maintenance of high metabolic activity, retention of body mass, and prevention of nutritional deficiencies, by providing a steady supply of nutrients to the digestive tract of each horse.

Seahorse Locomotion
Another reason that the seemingly simple question of stocking density poses such a problem is that seahorses are somewhat lazy hunters.  The more they must forage for food, the more energy seahorses must expend, and in a large, sparsely stocked tank, this can pose a serious problem for maintaining the health of your herd.  There seems to be a natural, built-in meter that tells them when enough is enough, such that if a seahorse does not find food after expending a certain amount of energy, he or she will stop hunting and find a place to hitch.  In this "energy-conservation mode," the horse will wait patiently for something edible to happen by so that very little effort must be given in order to feed.  This is especially true in tanks with too much current.  The excess water flow causes the horses to over-exert while swimming (due to the fact that they are poor swimmers) and they, consequently, exhaust themselves.  Further compounding the issue, the energy expended to "snick" up food is also significant, so if the horses are tired from swimming around in heavy current chasing their dinner, they may not have the energy to eat what they have finally caught up to! 

Now, imagine if you will, a single four-inch-long seahorse in a 20-gallon tank with pumps and filters blowing.  Those 20 gallons are a lot of territory to cover each time that little horse wants to eat.  It would be similar to you living in a 10,000-square-foot home, and whenever you were hungry, having to search every square inch of the place for a meal.  Couple this with the aforementioned physiological handicap relating to nutrition, and you can see how keeping your herd well and properly fed can be quite a dilemma.  As a seahorse keeper, you may opt to target feed your horse(s), but that can rapidly become inconvenient, does not give the fish a chance to actually hunt, and is not a guarantee that the horses will get to the food before it is swept away.  As an alternative, I recommend the proper stocking density for your tank dimensions and volume.

Stocking Density
The reason that this ratio is important and can alleviate the previously addressed issues is that seahorses will often take cues from each other regarding feeding, such that if one seahorse finds food and another hasn't, the one without food will move to where the food is.  The importance of this behavior increases in direct proportion to tank size.  The larger the tank, the more difficult it becomes to find food.  You don't want to overfeed the tank in order to make food more available to the horses because then water quality will suffer, so having more horses provides far greater chances of at least one finding food and notifying the other of where it is.  However, a delicate balance must be maintained between the number of seahorses housed and the size of the actual tank.  You will want to avoid overstocking your tank, but to a certain point, more seahorses are better.  As a rule of thumb, a 25-gallon "tall" tank (24" long x 12.5" wide x 20" tall) will house four to 14 horses, depending on species, size, and the desired stocking level.  Bear in mind, though, the more room you give them the happier they will be, so it truly is a tricky balance between space and stocking density.

Table 1, below, gives some tank-size-to-stocking ratios using common tank volumes and dimensions.  The figures are a result of data collected from many successful keepers and aquaculturing facilities:

Table 1

Tank Size (in gallons)	Seahorse Size	Minimum Recommended Stocking Capacity*	Optimal Stocking Capacity**	Maximum Recommended Stocking Capacity***
2	Mini	4	6-8	10
	Small	Larger species not recommended for this size tank
	Medium
	Large
10 Note: This tank size will be too shallow to allow any species other than minis to breed	Mini	12	16	20
	Small	4	6	8
	Medium	2	4	6
	Large	Not recommended for this tank size
25 tall	Mini	Miniature species not recommended for tanks of this size and larger
	Small	10	12	14
	Medium	6	8	10
	Large	4	6	8
39	Small	12	16	20
	Medium	10	14	18
	Large	8	10	12
55	Small	16	20	24
	Medium	14	16	18
	Large	10	12	14
75	Small	24	30	36
	Medium	18	22	30
	Large	14	16	18
90	Small	Small species not recommended for tanks of this size and larger
	Medium	22	26	30
	Large	14	16	20
125	Medium	28	32	36
	Large	16	18	20
150	Medium	30	34	38
	Large	18	20	22

Table 2

"Miniature" species (<4cm)	Small species (5-9cm)	Medium species (10-17cm)	Large species (>17cm)
H. bargibanti H. lichtensteinii H. minotaur H. zostrae	H. breviceps H. fisheri H. mohnikei H. sindonis H. zebra	H. angustus H. barbouri H. borboniensis H. camelopardalis H. capensis H. comes H. coronatus H. fuscus H. hippocampus H. histrix H. jayakari H. kuda H. spinosissimus H. trimaculatus H. whitei	H. abdominalis H. algiricus H. erectus H. guttulatus H. ingens H. kelloggi H. reidi H. subelongatus

*Minimum stocking capacity indicates the number of horses to keep while providing adequate chances of finding food.
**Optimal stocking capacity indicates the proper volume to stocking density ratio.
***Maximum stocking capacity indicates the highest number of horses to keep in the given volume of water in order to minimize stress and over competition for space and food, and not over-compromise water quality.

Afterword
These levels are educated approximations.  You will probably be safe in altering them by 5% to 10% in either direction in order to meet your needs or for aesthetic reasons.  Also, I am quite confident that there are many of you out there that will have different experiences with your horses.  I have had many over the years that have hunted voraciously until they bulged at the sides after every meal.  Contrarily, I have also had horses that never hunted, per se, preferring to lie in ambush for anything to float by.  The point is that the behaviors I have cited are typical behaviors of most horses.  Just as in life, there will always be exceptions and oddballs, but we should not let them determine the best course of action for the rest of the herd.

I hope this is useful to you all, and I wish you the best of luck with your seahorse corrals.