I’m sorry to hear that your seahorses are not feeling like their usual selves.
Some of the factors that are most often associated with increased lethargy and a loss of appetite are listed below:
(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.
To correct the situation, I suggest increasing the surface agitation and aeriation in your tank and combining 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.]
Regardless of how your water chemistry appears right now, a good place to start addressing this problem would be to perform a 25%-35% 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.
Whether the beneficial effects are due to improving water quality or replenishing depleted trace elements or something else altogether, performing a major water change and judicious aquarium cleaning as described above often sets things right when seahorses are off their feed for no apparent reason.
If the increased circulation and aeration, water changes and general aquarium cleaning don’t turn things around, then I would suggest treating your seahorse tank for parasites as previously described in the discussion on this page titled "new seahorse failure" in order to relieve the respiratory distress your seahorses are experiencing. Just go to the following URL and follow the recommendations outlined therein:
It would also be helpful if you can describe your current feeding regimen for me. What are you feeding your seahorses, how often are you feeding them, and how are you presenting the food to them (e.g., target feeding, scatter feeding, or using a feeding station)?
Best of luck restoring your seahorses’ appetite and getting things back to normal again, Hamiltoe.