Maximum Tank

Dear Moon Valley:

It’s great to hear that your seahorse tank has been going strong for two years now and that your herd of seahorses are thriving! It sounds like you have a fascinating aquarium and the fact that your Mandarin goby is still fat and sassy means that you are diligent aquarist and that your system must be providing an abundance of copepods and amphipods, which is ideal for your seahorses as well as the Mandarin. Well done! Keep up the good work!

In actual practice, 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).

Farm-raised 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).

Here’s what I can tell you for sure regarding the carrying capacity of your aquarium, Moon Valley. The recommended stocking density for large seahorses such as Mustangs and Sunbursts (Hippocampus erectus) in a tank like yours is about one pair per 10 gallons of water volume. So a reasonable number of average size Mustangs (or Sunbursts) to keep in a 40-gallon aquarium, for example, is a total of about 4 pairs or 8 individuals. An experienced seahorse keeper with a relatively sophisticated filtration system could comfortably keep around 10 H. erectus in such a tank with no problems, but a beginner with a basic filtration system should keep no more than 4-6 erectus in a tank that size, at least until he or she games a little more valuable firsthand experience keeping seahorses. With nine seahorses plus a Mandarin goby in your aquarium, I would say your tank is already stocked to capacity.

In your case, Moon Valley, the aquarium sump does makes a difference for the better and must also be taken into consideration when considering the maximum stocking density for your aquarium. Having your 40-gallon tank equipped with a 25-gallon sump may allow you to keep an extra pair or two of ponies, considering that you are an experienced seahorse keeper, but since your tank is operating smoothly with no problems and has been stable for a long time now, it would be a shame to press your luck by pushing the limits of your aquarium system any further.

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.

In short, Moon Valley, if you are eager to get some more seahorses, it may be time to consider starting up a new tank to accommodate them. If you are determined to add some more specimens to your 40-gallon aquarium, consider a few colorful invertebrates such as decorative shrimp or Fromia starfish, rather than any more ponies.

Best wishes with all your fishes!

Respectfully,
Pete Giwojna