It’s very difficult to determine what may be wrong with your little girl based on such sketchy information but I would be happy to share my thoughts on the matter with you. It would be helpful if you could give me a little more information about your tank in the meantime.
A 5-gallon aquarium is really too small for any seahorses other than Pixies or dwarf seahorses (Hippocampus zosterae), so I assume your little girl is a female dwarf seahorse. Is that correct? How long as the aquarium been set up, Marina? What are your current aquarium parameters (temperature, pH, specific gravity or salinity, and the levels of ammonia, nitrite and nitrate)? What type of filtration does your five-gallon tank use? Are there any fish or tankmates in the aquarium other than seahorses?
You did very well to perform a series of water changes, Marina. Because diseases are so often directly related to water quality, or due to stress resulting from a decline in water quality, when trouble arises the first thing you should do is to break out your test kits and check your water chemistry. Very often that will provide a clue to the problem. Make sure the aquarium temperature is within the acceptable range and check for ammonia and/or nitrite spikes first. See if your nitrate levels have risen to harmful levels and look for a drop in pH.
If any of your water quality parameters are off significantly, that may well be the cause of the problem or at least the source of the stress that weakened your seahorses and made them susceptible to disease. And correcting your water chemistry may well nip the problem in the bud, particularly if it is environmental, without the need for any further treatment.
Clean Up & Perform a Water Change
After a quick check of the water chemistry to assess the situation, it’s time to change water and clean up. In most cases, the surest way to improve your water quality and correct the water chemistry is to combine a 25%-50% water change with a thorough aquarium clean up. Siphon around the base of your rockwork and decorations, vacuum the top 1/2 inch of the sand or gravel, rinse or replace your prefilter, and administer a general system cleaning. The idea is to remove any accumulated excess organic material in the sand/gravel bed, top of the filter, or tank that could degrade your water quality, serve as a breeding ground for bacteria or a reservoir for disease, or otherwise be stressing your seahorses. [Note: when cleaning the filter, your goal is to remove excess organic wastes WITHOUT disturbing the balance of the nitrifying bacteria. Do not dismantle the entire filter, overhaul your entire filter system in one fell swoop, or clean your primary filtration system too zealously or you may impair your biological filtration.]
At first glance your aquarium parameters may look great, but there are some water quality issues that are difficult to detect with standard tests, such as a decrease in dissolved 02, transitory ammonia/nitrite spikes following a heavy feeding, pH drift, or the gradual accumulation of detritus. A water change and cleanup is a simple preventative measure that can help defuse those kinds of hidden factors before they become a problem and stress out your seahorses. These simple measures may restore your water quality and correct the source of the stress before your seahorse becomes seriously ill and requires treatment.
So a series of water changes was an excellent first step towards addressing this problem, Marina. Methylene blue is an excellent medication whenever fish are experiencing respiratory distress, so you also did the right thing in treating your little girl with methylene blue next to help ease her breathing. Just remember, though, that methylene blue should be used in a hospital tank, or as a brief dip or bath for the affected fish, rather than adding it to the main tank. This is because methylene blue can disrupt the biological filtration in your aquarium, which would cause a whole new set of problems.
If if the water changes didn’t help, and you recently, lost two fish from your tank shortly after adding a new acquisition that arrived ill, my best guess is that your little girl’s problem is not due to an environmental disease but rather to parasites that were introduced to the aquarium with the ailing fish. There are a number of very common ectoparasites that attack the skin and gills of fish, such as Uronema, Cryptocaryon, and Amblyoodinium, and these parasites typically enter the aquarium on a fish obtained from your LFS.
Without knowing more about your setup, and keeping in mind the materials you are likely to have on hand right now, the best thing I can suggest that this point would be to treat your little girl with a freshwater dip followed by a very brief dip in concentrated methylene blue. The freshwater dip will eliminate many of the parasites infesting the gills of your seahorse, thereby providing her with some immediate relief, while the methylene blue will further aid respirations. My best advice would be to proceed as follows, Marina:
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 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.
For best results, consider following up the 10-minute freshwater dip with a very brief (5-10 seconds — no more than 10 seconds maximum) dip in a solution of methylene blue between 30-50 ppm, as described below. Prepare the solution of methylene blue using saltwater from your seahorses tank ahead of time, before you do the freshwater dip, so you can dip them in the meth blue for 5-10 seconds right after you do the freshwater dip. Time the very short Methylene blue dip closely — maybe keep each seahorse in your hand while you dip it in the blue so there’s no fumbling around to capture it when time’s up — pull the pony out after 5-10 seconds and immediately return it to the main tank afterwards.
If you can obtain the Kordon brand of Methylene Blue (available at most well-stocked local fish stores), their suggested treatment protocol for treating external parasites as a dip is as follows:
For use as a dip for treatment of external parasitic protozoans:
(a) Prepare a nonmetallic container of sufficient size to contain the fish to be treated by adding water similar to the original aquarium.
(b) Add 5 teaspoons (24.65 ml) per 3 gallons of water. This produces a concentration of 50 ppm. It is not recommended that the concentration be increased beyond 50 ppm.
(c) Place fishes to be treated in this solution for no longer than 10 seconds.
(d) Return fish to original aquarium.
See the following link for more information on treating with Kordon’s Methylene Blue:
Click here: KPD-28 Methylene Blue
If your methylene blue is not Kordon (KPD-28) Methylene Blue, then disregard the instructions above and follow the manufacturer’s instructions for using your brand as a bath or dip instead. Please let me know how your little girl tolerates these first aid measures, Marina, and will proceed from there…
Best of luck resolving this problem, Marina! Once I know a little more about your seahorse setup, I may be able to provide you with some better advice and additional treatment options.