A number of seahorse species have a color pattern that includes small dark spots scattered profusely over their bodies. For example, this is typical of Hippocampus reidi, most H.kuda, and some H. barbouri. Mustangs and Sunbursts (H. erectus) normally have a lined pattern underlying their base coloration, rather than spots, although many specimens have tiny white and/or black spots as well.
So it’s possible the black dots may be natural markings that only show up at certain times when the seahorse is exhibiting sufficiently light background or base colors. It’s also possible that the black specks could be ectoparasites of some sort, but in that case I would expect the same black parasites to be infesting all the seahorses in the tank, and I would expect the Sunburst with the black dots to show other symptoms or signs of irritation such as rapid breathing, respiratory distress or scratching.
If you are confident the black dots aren’t natural markings or a transitory color phase, you may want to consider performing a prophylactic freshwater dip as a precaution. If done properly, according to the following directions, a freshwater dip is a very safe procedure for seahorses, and if the black dots are ectoparasites, the dip will cause many of them to lyse and drop off there host. They will then be left behind in the dipping water, providing clear evidence of a parasite problem.
A freshwater water dip is simply immersing your seahorse in pure, detoxified freshwater that’s been preadjusted to the same temp and pH as the water the seahorse is accustomed to, for a period of at least 10 minutes (Giwojna, Dec. 2003). It doesn’t harm them — seahorses typically tolerate freshwater dips exceptionally well and a 10-minute dip should be perfectly safe. Freshwater dips are effective because marine fish tolerate the immersion in freshwater far better than the external parasites they play host to; the change in osmotic pressure kills or incapacitates such microorganisms within 7-8 minutes (Giwojna, Dec. 2003). A minimum dip, if the fish seems to be doing fine, is therefore 8 minutes. Include some sort of hitching post in the dipping container and shoot for the full 10 minutes with your seahorses (Giwojna, Dec. 2003).
If you will be using tap water for the freshwater dip, be sure to dechlorinate it beforehand. This can be accomplished usually one of the commercial dechlorinators, which typically include sodium thiosulfate and perhaps a chloramine remover as well, or by aerating the tap water for at least 24 hours to dissipate the chlorine (Giwojna, Dec. 2003).
If you dechlorinate the dip water with a sodium thiosulfate product, be sure to use an airstone to aerate it for at least one hour before administering the dip. This is because the sodium thiosulfate depletes the water of oxygen and the dip water must therefore be oxygenated before its suitable for your seahorse(s).
Observe the horse closely during the dip. You may see some immediate signs of distress or shock. Sometimes the horse will immediately lie on its side on the bottom. That’s a fairly common reaction — normal and to be expected, rather than a cause for concern, so don’t be alarmed if this happens. Just nudge or tap the seahorse gently with your finger if it lies down on its side. Normally, the seahorse will respond to the slight nudge by righting itself again and calm down for the duration of the dip. However, if it does not respond, stop the treatment.
Most seahorses tolerate the treatment well and experience no problems, but if you see continued signs of distress — twitching, thrashing around, etc. — stop the treatment.
After you have completed the dip and returned the seahorses to the aquarium, save the dip water and examined it closely for any sign of parasites. The change in osmotic pressure from saltwater to freshwater will cause ectoparasites to lyse (i.e., swell and burst) or drop off their host after 7-10 minutes, and they will be left behind in the dipping water. Protozoan parasites are microscopic and won’t be visible to the naked eye, but some of the other ectoparasites can be clearly seen. For example, monogenetic trematodes will appear as opaque sesame seeds drifting in the water (Giwojna, Aug. 2003) and nematodes may be visible as tiny hairlike worms 1/16-3/16 of an inch long. Other parasites may appear as tiny dots in the water. Freshwater dips can thus often provide affected seahorses with some immediate relief by ridding them of these irritating pests and can also aid their breathing by flushing out gill parasites.
Best of luck with your seahorses, Haynes! If you feel a freshwater dip is warranted, please let us know right away if it reveals any signs of parasites.