Re:Tank Size – How Many Pairs

Dear Lelia:

The short answer to your question is yes — your new 57-gallon aquarium certainly has the carrying capacity to safely support a couple pairs of Sunbursts in addition to your breeding pair of Mustangs. But you are interested in determining the optimum stocking density for your new system, so let’s discuss some of the factors that determine the carrying capacity for any given aquarium in greater detail.

In actual practice, Lelia, determining how many seahorses can comfortably live in an aquarium of a certain size is not as simple as it seems at first glance. The proper stocking density for any given setup depends on a great many complex factors. I have listed a few of the most important of these below:

· The size of the aquarium.
· The filtration system it uses.
· Is it a species tank or a mixed community?
· The number and type of non-seahorse tankmates it houses.
· The type of seahorses you will be keeping and the maximum size they reach.
· The experience level of the seahorse keeper.
· Are the seahorses you will be keeping wild specimens or farm-raised livestock?

Many of the considerations you must keep in mind when stocking your aquarium are self-explanatory. For example, common sense dictates that the bigger the tank the more seahorses it can safely house, or that an aquarium of given size can support more small to medium sized seahorses than it can if stocked with one of the giant breeds. And you don’t need to be Jacques Cousteau to realize that if you are keeping your ponies in a mixed community with other reef fishes, you will have to settle for fewer Hippocampines than if you kept them in a species tank dedicated to seahorses only (Giwojna, Jan. 2002).

Likewise, the experience level of the hobbyist certainly has a bearing on how many seahorses he should attempt to keep in a given volume of water. If you’re a rank beginner, you will be better off keeping your stable under stocked in order to provide a margin of error while you learn the ropes with these amazing aquatic equines. Savvy seahorse pros who’ve seen it all before and know all the tricks and trouble spots, on the other hand, can afford to push the envelope a bit and keep their herds near capacity (Giwojna, Jan. 2002).

In addition, the filtration system obviously affects the number of specimens a certain aquarium can support, yet it is often overlooked when stocking densities are discussed. Consider two identical 29-gallon (tall) tanks: one relies on undergravels and/or foam filters, perhaps supplemented by a small external, hang-on-the-back filter packed with media such as activated carbon; the other features plenty of live rock and perhaps even a live sand bed, supplemented with a good protein skimmer and a power filter for added circulation and water movement. The first simple setup has an adequate biofilter but is something of a nitrate factory, whereas the more sophisticated setup has significant dentrification ability in addition to plenty of biofiltration (Giwojna, Jan. 2002). Both systems have the right dimensions and sufficient water volume to support several large seahorses, but you don’t need to be a marine biologist to understand that the live rock setup with the skimmer can handle a greater bioload and safely house more specimens than the more basic system (Giwojna, Jan. 2002).

Perhaps the most common mistake seahorse keepers make when considering the appropriate stocking density for their systems is failing to distinguish between wild-caught and captive-bred seahorses. Enough field work and research has now been done to conclude that, in terms of their behavior and need for elbow room, seahorses in the wild are very different animals from captive-bred and raised seahorses (Giwojna, Jan. 2002). For example, field studies show that pair-bonded seahorses typically enjoy a large territory in the wild (100 square meters in the case of female Hippocampus whitei, a fairly small Australian species that has been studied closely), and with their patchy distribution pattern, these seahorses only infrequently come in contact with others of their kind (Vincent & Sadler, 1995). Traumatic capture techniques, mishandling, and lack of feeding opportunities often plague wild-caught seahorses during transport and holding, and by the time they finally arrive at your local dealer’s, chances are great that wild ponies have already endured quite an ordeal (Bull and Mitchell, 2002). Malnutrition and stress at a time of high metabolic demand are likely to have weakened them (Lidster, 2003). When confined in an aquarium, therefore, wild-caught horses do not tolerate crowding well, and given their low disease resistance compared to their captive-bred brethren, it is NEVER a good idea to crowd wild-caught seahorses. They often have a more difficult time acclimating to life in captivity and will therefore be stressed, at least initially (Giwojna, Jan. 2002).

Domesticated seahorses, on the other hand, are raised at far greater population densities than any seahorse experiences in the wild. Born and bred for aquarium life, they are far more social than wild caughts and are used to living in close proximity to each other (Giwojna, Jan. 2002). For them, that’s their normal condition and the aquarium is their natural environment. They reach the hobbyist well fed, in peak condition, and already accustomed to aquarium life and frozen foods. As a result, farm-raised seahorses are simply hardier, more disease resistant, and tolerate crowding and life in captivity far better than their wild-caught counterparts (Giwojna, Jan. 2002). Suffice it to say that more captive-bred seahorses can be maintained in an aquarium of a given size than wild-caught ‘horses.

Quantifying all of this, and specifying a certain number of seahorses per so many gallons of water, is a very tricky proposition because so many factors like those described above must be weighed. Consequently, my recommendations for stocking density always include a range for each size of aquarium in order to accommodate variables such as differing filtration systems, whether the seahorses are wild or captive bred, and varying levels of expertise. If you’re new to seahorses or have a basic setup that relies on regular partial water changes to control nitrates, you will need to stick to the lower end of the recommended range when stocking your stable (Giwojna, Jan. 2002). However, if you’re an experienced reefer or an old hand at seahorse wrangling, with a relatively sophisticated system at your disposal, feel free to explore the upper limits of the suggested stocking densities (Giwojna, Jan. 2002). Likewise, if you’re keeping wild-caught seahorses, I suggest you cut the recommended stocking densities for captive-bred seahorses at least in half (Giwojna, Jan. 2002).

Assuming that your aquarium will be a dedicated seahorse tank and not a community tank, and that you’ll be keeping captive-bred seahorses such as Mustangs or Sunbursts of average size, Lelia, the suggested stocking density for Hippocampus erectus under those circumstances is about one pair per 10 gallons of water volume. So a reasonable number of average size Mustangs (or Sunbursts) to keep in a 57-gallon aquarium, is a total of about five or six pairs or 10-12 individuals. An experienced seahorse keeper with a relatively sophisticated filtration system could easily keep a dozen H. erectus in such a tank with no problems, but a beginner with a basic filtration system should keep no more than about six erectus in a tank that size, at least until he or she games a little more valuable firsthand experience keeping seahorses.

In your case, Lelia, since you are new to seahorses, I would suggest adding two pairs of Sunbursts to go with your breeding pair of Mustangs for starters. Then, after you’ve gained a little more valuable first-hand experience with seahorses, your 57-gallon seahorse setup has the volume and stability to safely house one or two more pairs of seahorses. But I would not ever exceed a total of 10 seahorses in that system, and I would limit myself to four pairs or eight individuals to provide a margin for error at all times. For best results, you should build up your herd gradually in order to allow the biofiltration plenty of time time to adjust to the heavier bioload after you make new additions without spikes in your ammonia or nitrite levels.

And, as always, be sure to remember the three golden rules that should always guide your actions when stocking any seahorse setup:

I. Under stocking is ALWAYS better than over stocking. Always! That is the one immutable law that governs the seahorse-keeping universe, and if you violate it, the aquarium gods will exact swift and terrible retribution!

II. When in doubt, under stock. Don’t push your luck! If you have any doubt whatsoever as to whether or not your system is running at capacity, it probably is. In such a situation, you MUST err on the side of caution.

III. Don’t mess with success! If your seahorse setup has been running smoothly and trouble-free for a prolonged period at it’s present level of occupancy, try to resist the temptation to increase your herd. Why risk upsetting the balance in a system that has settled into a state of happy equilibrium? Rather than risk overcrowding an established tank, consider starting up a new aquarium when the urge to acquire some new specimens becomes overwhelming.

When stocking your aquarium, consider these golden rules to be your commandments. Obey them, and your system should flourish. Surely goodness and mercy shall follow thee all of thy days. Break them, and you will soon find yourself teetering on the brink of disaster. Abandon all hope ye whom embark down that dark road to ruin.

Best of luck with your new 57-gallon seahorse setup, Lelia! Feel free to expand your herd by two or three pairs when the time is right.

Happy Trails!
Pete Giwojna