It’s very difficult to say for certain why one of your seahorses is periodically scratching itself, especially without knowing more about your seahorse setup, your water quality parameters, and any other tankmates you may have.
However, external parasites are one of the most common causes of skin irritation that results in the seahorses scratching themselves against objects in the aquarium, and such external parasites are especially common in seahorses that have been collected from the wild or in ponies obtained from pet shops and fish stores.
The periodic but persistent scratching that one of your seahorses is doing does indeed suggest a problem with an infestation of such ectoparasites, most likely protozoan parasites (ciliates) of some sort. As a treatment option, I would suggest first administering a freshwater dip to the affected seahorses to provide them with immediate relief, followed by treating your seahorse tank with a good antiparasitic medications such as praziquantel, as explained below in more detail, Lora.
In order to treat an infestation of external protozoan parasites, the whole tank must be treated to eliminate the parasites once and for all. Praziquantel is a good treatment option that would accomplish that goal without affecting the beneficial nitrifying bacteria that are providing the biological filtration for the aquarium.
If the scratching you are concerned about is due to ectoparasites, Lora, a fresh water dip is a good first aid measure to begin with. It is a good idea to provide the seahorses that are scratching with some immediate relief from the parasites by dipping them in freshwater
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 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 experienced 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 seahorse(s) 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.
Here is some additional information about the praziquantel antiparasitic medication that I recommend you treat your seahorse tank with after providing your ponies with some quick relief via a fresh water dip, Lora:
This is a very effective antiparasitic that works equally well against external and internal parasites alike. Like metronidazole, this is a very safe medication that won’t harm the beneficial nitrifying bacteria in your biofilter, so you can use it to treat the main tank.
However, like most antiparasitic medications, invertebrates will not tolerate praziquantel, so be prepared to relocate any snails, decorative shrimp, live corals, or microhermit crabs for the duration of the treatments.
Praziquantel can be administered orally via bio-encapsulated feeder shrimp, just like metronidazole, or it can be administered as a series of baths or as a one-time treatment for the main tank. When using it as a bath, the following concentrations are appropriate:
Praziquantel bath at 10ppm for 3 hours or 1ppm for 24 hours.
When treating ectoparasites, it can be added directly to the water in the treatment tank (dose one time and leave in the water for 5-7 days).
You should remove any activated carbon or chemical filtration media during the treatments, of course.
As I mentioned, the praziquantel won’t harm the beneficial nitrifying bacteria or affect the biological filtration, but it can be hard on invertebrates so it would be a good idea for you to remove all of the snails and other decorative invertebrates (just keep them in a clean, well-aerated plastic bucket of saltwater during the treatment period for safekeeping.)
If you are lucky, you may be able to find the praziquantel at one of your local fish store, but you may have to get it online and have it delivered to you, Lora. You can obtain praziquantel from National Fish Pharmaceuticals at the following website (just copy the following URL and paste it in your Web browser and it will take you to the right webpage):
National Fish Pharmaceuticals also has a frees free helpline (520-298-7814) that you can telephone for assistance, if necessary.
Best of luck relieving the itching and returning your ponies to normal again, Lora!
Pete Giwojna, Ocean Rider Tech Support