The recommended stocking density for large tropical seahorses, such as Ocean Rider Mustangs and Sunbursts (Hippocampus erectus), is one pair per 10 gallons, with a minimum tank size of 30 gallons.
Therefore, if your 38-gallon aquarium were a dedicated seahorse setup that was equipped with an efficient filtration system, sir, it could safely accommodate around 4-5 pairs of seahorses or 9-10 adult individual Hippocampus erectus when it was stocked to capacity.
However, it’s always better to keep your aquarium under stocked in order to provide a margin for error, and that’s especially important if you are a beginner and these will be your first seahorses.
And, of course, your 38-gallon aquarium is going to be a community tank that includes seahorses, rather than a dedicated seahorse setup. The other tropical fish you mentioned – a pair of “Nemo” false percula clownfish (Amphiprion occelaris), a pair of red banded pipefish (Doryrhamphus dactyliophorus), and a pair of Banggai Cardinals (Pterapogon kauderni) – will all make compatible tankmates for the ponies but their presence will naturally reduce the number of seahorses you can safely keep in the aquarium.
As a rough rule of thumb, you should cancel out one seahorse for each of the other small fish the aquarium houses when considering the appropriate stocking density, Chase, which would mean that the 38-gallon community tank could actually accommodate perhaps 3-4 Hippocampus erectus seahorses when stocked at capacity, rather than 9-10 of them.
And, in order to allow yourself a comfortable margin for error, I would say it would be best to limit yourself to no more than one pair of Mustangs or Sunbursts to go along with the other specimens you listed.
Also, sir, I should point out that a turnover rate of 21 times per hour will produce powerful water currents that are too strong for the limited swimming ability of the seahorses, and you will therefore need to reduce the water flow in the aquarium one way or another before you consider introducing any seahorses.
This is what I normally advise home hobbyists regarding the water movement in a seahorse setup, Chase:
Water Circulation for the Seahorse Tank
Many seahorse keepers are overly conscious of the inactive life style and limited swimming ability of Hippocampus, and have adjusted their flow rates accordingly, resulting in undercirculated tanks with too little water movement. That’s a serious mistake for a small, close-system aquarium.
In actuality, seahorses prefer moderate water movement, including some areas of brisk current, providing there are also sheltered spots and some areas of relatively slack water they can move to when desired. Slack water means comparatively low flow, NOT stagnant conditions! As with any aquarium, avoid dead spots and stagnant areas in the seahorse tank at all costs (Giwojna, unpublished text).
Contrary to popular opinion, seahorses are quite effective swimmers that can hold their own in strong currents as long as sheltered areas are available (Delbeek, Oct. 2001). I have often discussed this matter with professional divers and collectors who regularly encounter seahorses in the ocean, and they report that the horses are often found where you would least expect them — well offshore and thriving in areas with powerful currents. For example, here is how Paul Baldassano, a commercial diver in New York who makes his living collecting sea urchins, describes the behavior of his local seahorses:
“In regard to seahorses in the wild, I occasionally see Hippocampus erectus in the wild while SCUBA diving but never in the places where they are supposed to be. I see them in the open sea far from shore and also in areas with large rocks and very strong currents. The last one I saw was in a channel off the south shore of Long Island New York in water about 12 feet deep. The current was so strong that I had to hold on to the rocks so as not to be swept away. This Hippocampus erectus was having no trouble staying there munching on the abundant plankton. Apparently they find places near the rocks where there is no current because as you know they are lousy swimmers. There is also a large population of seahorses in a similar area in another part of the New York shore, but I think it is best not to divulge that location for obvious reasons (Baldassano, pers. com.).”
Neil Garrick-Maidment, a very successful seahorse breeder in the UK, reports much the same thing, noting that seahorses in the wild seem to thrive amid strong currents:
“Whenever I have dived on Seahorse sites I have always been amazed by the currents and tides that this very fragile looking Seahorse lives in. We often find Seahorses in flat muddy/silt areas nowhere near rocks or weed. These areas are often scoured by strong currents and the Seahorses do well in them and seem completely unperturbed by the current (Garrick-Maidment, pers. com.). In setting up a tank for them I try to remember the feeling I had in those areas and replicate them. I have now started to use wave surge devices, so that the current in the tank, although strong (they seem to thrive in strong currents) varies in its direction (Garrick-Maidment, Jun. 2002).”
Kirk Strawn, who earned his Master of Science thesis studying Hippocampus zosterae in the field, echoes Neil’s thoughts on the matter:
“The aquarist is not giving his seahorses natural conditions when he keeps them in a still-water aquarium. In nature tidal currents, wind, and waves are usually mixing the well aerated surface film water with the deeper water.”
Likewise, David Warland, a fish farmer and commercial seahorse breeder in Port Lincoln, Australia, reports he often finds Hippocampus abdominalis perching on the tuna net enclosures at the farm in deep water:
“The Horses that are around the farms have traveled vast distances over plain sand/mud to get to the farms, which are in at least 20 meters of water, and are miles from the nearest land or shallow water (Warland, pers. com.).”
And Jorge Gomezjurado, the Senior Aquarist at the National Aquarium in Baltimore, recommends the following when it comes to water movement:
“I personally believe that current and water dynamics are very important for Syngnathids. In nature they live in areas with active water movement.(i.e., tides in mangrove lagoons and estuaries, coral reefs, kelp forests, etc.). Why don’t we give them the same environmental conditions in captivity? Our small tanks (90 gallons) have large turnovers on an average of 5 gpm (or 300 gph). It is very important that the current is steady and directionally constant, which allows the animals to find a good spot to hold and they will not be pulling in different directions all the time.”
Most seahorse keepers feel it’s best to keep the current steady and nonvarying so they can find slack-water areas and sheltered spots downcurrent to hold in when they want to get out of the current. The more brisk the water flow, the more important this becomes. However, in a large aquarium with low to moderate water movement, alternating currents should not present much of a problem, and would help to provide good circulation throughout the tank.
You’ll want to adjust the outflow the filters to eliminate any dead spots or stagnant areas where waste products may tend to accumulate. Good circulation will prevent pockets of harmful anaerobic decay and keep particulate matter suspended in the water column where the filters can remove it from the aquarium. Alternating the direction of the water flow is also helpful, as is increasing surface agitation to improve the oxygenation and facilitate more efficient gas exchange at the air/water interface. A simple air stone anchored just beneath the surface of the water can help to achieve this goal.
The point is that, as long as slack-water retreats are available, the greater seahorses can tolerate far more current than most folks suspect and good circulation is as important for a seahorse setup as any other aquarium. What seahorses lack as swimmers is not agility, but rather stamina (Evans, 1998). They can hold their own against strong currents, but not indefinitely, so low flow areas where they can move out of the current and hold when they want to rest must be provided in addition to good circulation.
In short, if your filtration is not turning over the entire volume of the aquarium a MINIMUM of 5 times per hour, your seahorse setup is undercirculated. With a spray bar return raised above the surface of the water to diffuse the outflow, you can safely achieved much higher turnover rates (> 10 times per hour) without producing too much turbulence or current for seahorses in a tall tank. A waterfall return is another good way to diffuse the output from your filter, and also works well for seahorses. There will be an area of relatively vigorous water movement at one end of the aquarium underneath and nearby the waterfall, while the other end of the tank is a relatively low flow area. (By the same token, however, if the filtration system in your seahorse tank is turning over the entire volume of water much more than five times per hour, it may be too overpowering for the seahorses unless it is diffused by a spray bar or waterfall return.)
As with anything, too much of even a good thing can be undesirable, and too much current can overwhelm the limited swimming ability of Hippocampus. One indication that you may have too much water movement in your seahorse tank is if the seahorses are getting buffeted around by the currents, and whisked away uncontrollably when they tire of fighting the current. Or alternatively, they may stay perched in one place all the time and refuse to swim around and explore their tank for fear of getting swept away by the current if they relax their grip on their hitching posts. So you can get a pretty good gauge of how well the seahorses are able to cope with the water movement than their tank by observing how the current affects the swimming ability.
Likewise, if a mated pair of seahorses is consistently spilling eggs during the copulatory rise, that’s another pretty good indication that there may be too much turbulence or water movement in the upper reaches of their aquarium.
If the seahorses are having difficulty tracking their prey and eating because the current whisks the frozen Mysis past them too quickly to target it accurately and slurp it up, that’s another red flag. Often that situation can be corrected simply by adjusting the output from your filter to reduce the current during feeding time or turning it off altogether while the seahorses are eating.
But as long as your seahorses aren’t getting buffeted around, aren’t routinely dropping eggs during disrupted mating attempts, and aren’t having difficulty targeting their prey and eating, there’s really no such thing as too much water movement. In general, the stronger the water flow, the more important it is to keep the water currents steady and unvarying so the seahorses can establish holding areas in the sheltered spots and low-flow zones without getting blindsided by unpredictable currents. Just make sure your seahorses are not getting trapped against overflows and be sure to screen off the intakes for any powerheads. Powerheads can be switched off at feeding time, if necessary.
Okay, Chase, that’s the rundown on water circulation in a seahorse setup. I recommend a turnover rate of 5-7 times per hour for a seahorse setup, generally speaking, so in your case, you want the filters for the aquarium to put out between 190-270 gallons per hour (190 gph/38 gallons = 5 times per hour; 270 gph/38 gallons = 7 times per hour).
In other words, sir, the filters you currently have on the aquarium will turn over the entire water volume of the aquarium 21 times per hour, which is about three times the recommended amount of water movement. Currents that strong would overpower the limited swimming ability of the ponies, so you are going to need to reduce the water flow or greatly soften and diffuse the water flow in order to make it suitable for seahorses.
Best wishes with all your fishes, Chase!
Pete Giwojna, Ocean Rider Tech Support