There are number of things you can do to deal with such a situation, Doki. First of all, you should perform a major water change regardless of how the water quality parameters check out, and you should also increase the surface agitation and aeration in your seahorse tank immediately to increase the levels of dissolved oxygen and decreased the levels of dissolved carbon dioxide in the water.
Here is a more detailed explanation of the steps you should take whenever one of your seahorse suddenly stops refusing food, Doki:
Loss of Appetite & Hunger Strikes
An unexplained loss of appetite in an otherwise healthy seahorse is also often an environmental problem. Many times such eating problems are due to low levels of dissolved oxygen or high levels of carbon dioxide, and they can frequently be caused by deteriorating water quality, especially deficiencies in certain minerals and trace elements. Lack of appetite is therefore often an early indicator of water quality problems.
When a seahorse goes off its feed, the first things to consider that will often help restore its appetite are to perform a series of water changes to restore water quality and to try tempting the seahorse with live foods, as discussed in greater detail below:
For starters, I have listed some of the factors that are commonly known to contribute to a loss of appetite in seahorses:
(1) deteriorating water quality.
(2) low oxygen and/or high CO2 levels.
(3) a deficiency of trace elements and minerals.
(4) various disease processes — in particular, internal parasites.
Regardless of how your water chemistry appears right now, a good place to start addressing loss of appetite is to one or more 25%-35% water changes immediately to safeguard the water quality and replenish depleted trace elements and minerals. (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, a deficiency and trace elements/minerals, 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 as well as your seahorses’ appetite.)
Be sure to check your dissolved oxygen (O2) level in addition to the usual pH, ammonia, nitrite and nitrite readings.. A significant drop in O2 levels (6 – 7 ppm is optimal) or rise in CO2 levels is very stressful yet easily corrected by increasing surface agitation and circulation to promote better oxygenation and gas exchange. Add a shallow airstone just beneath the surface if necessary and increase the circulation throughout your tank it possible.
Whether the beneficial effects are due to improving water quality or replenishing depleted trace elements or something else altogether, performing a major water change, or a series of water changes, as described above often sets things right when seahorses are off their feed for no apparent reason.
In the meantime, while you are working on your water quality, by all means get some live foods to tempt your finicky seahorse and see if you can fatten it up a bit. When a seahorse stops eating, the most important thing is to get some food into him one way or another. You’ve got to keep his strength up and give him a chance to recover before you can worry about weaning him back onto frozen foods again. Hawaiian red feeder shrimp or volcano shrimp (Halocaridina rubra) are ideal for this — seahorses find them utterly irresistible! But anything that’s readily available — enriched adult brine shrimp, live ghost shrimp that are small enough to be swallowed, newborn guppies or mollies, Gammarus amphipods, copepods, you name it — is worth a try. Just get some good meals into the reluctance seahorse ASAP and anyway you can to build up its strength and help it regain its conditioning.
The problem may simply be a hunger strike, especially if you are dealing with a wild-caught seahorse that is dependent on live foods. When a wild seahorse suddenly stops eating, many times it has simply lost interest in frozen foods. Although this is rarely a problem with domesticated seahorses that are accustomed to eating frozen foods from an early age, hunger strikes are common developments when keeping wild-caught seahorses. If so, providing live foods, at least temporarily, will often turn the situation around.
When seahorses tire of the same old, boring frozen food and refuse to eat their "veggies," living prey is what they crave: Mysids, feeder shrimp, Gammarus or adult Artemia — the type of food isn’t really as important as the fact that it’s alive and kicking. Nothing stimulates a sea horse’s feeding instincts like the frantic movements and evasive maneuvers of real, live, "catch-me-if-you-can" prey items (Giwojna, 1996).
That’s why I like to use occasional treats of live food as behavioral enrichment for my seahorses. They get the thrill of hunting after and chasing down live prey, which livens things up for them in more of ways than one and is a nice change of pace from their daily routine in captivity. Live foods are guaranteed to perk up an ailing appetite and excite the interest of the most jaded "galloping gourmets." When it comes to a hunger strike, living prey is the only sure cure for the "Bird’s Eye blues." (Giwojna, 1996)
I also find live foods to be especially useful for those rare occasions when seahorses are ailing and must be treated. Many medications (e.g., Diamox) have the unfortunate side effect of suppressing appetite, so when treating sickly seahorses, it’s a good idea to tempt them with choice live foods in order to keep them eating and help build up their strength while recuperating. Separating an ailing seahorse from its mate and herdmates and transferring it to a strange new environment for treatment can be a traumatic experience, especially since the Spartan surroundings in the sterile environment of a sparsely furnished hospital tank can leave a seahorses feeling vulnerable and exposed. Live foods can counteract these negative affects to a certain degree, and offer a little excitement that distracts the isolated seahorse temporarily at least from its melancholy.
Some of the choice live foods that sea horses find irresistible are Ocean Rider’s red feeder shrimp (Red Iron Horse Feed, Halocaridina rubra), Gammarus amphipods, and the live Mysis post-larval Feeder Shrimp from Drs. Foster and Smith (liveaquaria.com). These live bite-size crustaceans are what I’d like to call a "feed-and-forget" food. They are tough, rugged little shrimp that you can toss in your tank with no acclimation whatsoever. They are agile and elusive enough that your filters won’t eat them and the seahorses won’t be able to capture them all right away. Some will hide and evade well enough that your seahorses will still be hunting down the stragglers for the next day or two. Best of all, you can toss a nice batch of them in your aquarium, secure in the knowledge that they won’t perish and pollute it, but thrive and survive as real, live, "catch-me-if-you-can" prey items that seahorses cannot resist.
When a seahorse goes off its feed, providing it with choice live foods can buy you time and stave off starvation while you work on making the water changes to assure optimal water quality for your seahorses.
The Ocean Rider Aquaculture Facility in Hawaii (http://seahorse.com/) is a good source for the following live foods:
Green Iron Horse Feed (Gammarus amphipods)
Red Iron Horse Feed or Volcano Shrimp (Halocaridina rubra)
Or the live Mysis or post-larval Feeder Shrimp from Drs. Foster and Smith would also be a good choice for this. You can obtain 100 live Mysidopsis bahia for $33.99 or 100 bite-size Feeder Shrimp for $39.99 from liveaquaria.com and your seahorses will love them. Just copy the following URL (everything within the angle brackets below), paste it in your web browser, and press the "Entered" key, Doki, and it will take you directly to the right webpage:
Some hobbyists have good success coaxing a finicky seahorse to feed by transferring the seahorse to a critter keeper or breeder net or similar enclosure that can hang within the main tank itself, and then adding a generous portion of live feeder shrimp to the container. Within the enclosure, the affected seahorse does not have to compete with its tankmates for the live food, and it is easy to maintain an adequate feeding density within the confined space so that there is always a bite-size feeder shrimp passing within striking distance of the hungry seahorse. If the affected seahorse is still interested in feeding at all, then releasing it in an in-tank enclosure like this where it will be surrounded by plenty of tempting live feeder shrimp and can feed at its leisure may help it to keep its strength up and recover more quickly. Add one or two hitching posts within the critter keeper or breeder net so that the seahorse can anchor in place and wait for a tasty shrimp to pass within easy reach, and give him an hour or two within the enclosure to eat his fill of the feeder shrimp. You can monitor his progress from a nonthreatening distance away from the tank to see how he is doing. In most cases, the seahorse quickly becomes familiar with the routine of being transferred to the special enclosure at feeding time and associates it with tasty live foods and a full belly — positive reinforcements that make it a very nonthreatening, stress-free procedure for the affected seahorse — and, as a result, it may actually come to look forward to it after a few feedings. You can repeat this feeding process two or three times daily in order to fatten him up again, if your schedule allows.
If your seahorse’s loss of appetite is associated with a change in its fecal pellets, that could indicate a problem with internal parasites. For example, a change from fecal pellets of normal color and consistency to white, stringy mucoid feces accompanied by hunger strike is often an indication of intestinal flagellates (Kaptur, 2004). If you think that this could be a factor in your case, then treatment with metronidazole or praziquantel is usually an effective remedy (Kaptur, 2004).
Okay, that’s the quick rundown on some of the things you can do immediately to perk up your seahorse and restore her appetite to normal again, Doki. You should immediately begin a series of water changes and increase the surface agitation and oxygenation in your seahorse tank. Go ahead and install an airstone, air diffuser, air bar, or bubble wand in your seahorse tank positioned where the stream of bubbles will not be drawn into the intake for the filtration system and that may be all you need to do to resolve the situation for now. If necessary, lineup some choice live foods to tempt her to eat as well.
Best of luck resolving the situation and getting your seahorses back to normal again, Doki. Please update me to let me know if they respond to the water changes and increased oxygenation in the aquarium, or at least begin feeding live foods. If not, there are other things that you can do to get some badly nutrition into your ponies and fatten them up again.
Pete Giwojna, Ocean Rider Tech Support