I’m sorry to hear about the situation with your seahorses. If they are off their feed, have abandoned their feeding station, and have taken to hiding, it’s obvious that something is stressing them.
It’s very difficult to say what the source of that stress may be, but I can tell you that the factors that are most commonly associated with a loss of appetite are the following:
(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.
In your case, Amanda, I suspect it may be a water quality problem. I’m leaning in that direction for a couple of reasons. First of all, a 27-gallon aquarium housing six adult seahorses is stocked to capacity, which means there is not much margin for error and the water chemistry can go south in a hurry. Secondly, your tank has been having a bristleworm problem lately, and a population explosion of bristleworms goes hand in hand with an excess of nutrients and heavy nutrient loading in an aquarium. I can envision a scenario in which excess organics and detritus have been gradually accumulating in your crowded aquarium for the past six months and only just recently reached the level where they are problematic. Some aspect of your water chemistry may have deteriorated below some critical threshold level as a result and become a source of stress for the seahorses. This could be something quite subtle, entirely beyond the basic aquarium parameters you are measuring (ammonia, nitrite, nitrate, pH, and temperature), as discussed below:
At any rate, regardless of how your water chemistry appears right now, I would perform a a major water change 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.
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 and vacuuming the substrate, 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.]
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 two as described above often sets things right when seahorses are off their feed for no apparent reason. So that would be a good place for you to start when addressing this problem, Amanda.
In the meantime, while you are working on your water quality, I would switch from frozen foods to live foods temporarily. This would accomplish two things — tempting the seahorses to resume feeding aggressively, thereby getting some badly needed nourishment into them, and safeguarding your water quality. If the seahorses are no longer coming to their feeding stations, offering them frozen Mysis two or three times a day is likely only fattening up your bristleworms add the uneaten Mysis may also be further degrading your water quality. Live foods that can survive indefinitely in saltwater, such as enriched adult brine shrimp, Gammarus amphipods, red feeder shrimp from Hawaii, or post larval shrimp from http://www.seawaterexpress.com/ won’t go to waste or break down and pollute the tank. Plus the live prey will find its way to your seahorses when they are hiding amidst the rockwork and are therefore more likely to be eaten.
In short, I would perform a judicious aquarium cleaning along with one or more major water changes, and try tempting your seahorses with their favorite live foods, and see if that restores their appetite and helps things get back to normal. (Is your aquarium equipped with a protein skimmer, Amanda? If not, installing a protein skimmer might make a big difference in water quality and help restore the normal equilibrium in your aquarium again.) If that doesn’t help in the problem persists, let me know and there are one or two other things we can try to remedy the situation.
Here’s hoping your seahorses are shown feeling like their old selves again, Amanda.