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First sign of a health problem
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.
Be sure to check your dissolved oxygen (O2) level too. A significant drop in O2 levels (6 - 7 ppm is optimal) is very stressful yet easily corrected by increasing surface agitation and circulation to promote better oxygenation and gas exchange. At the other extreme, oxygen supersaturation is a red flag indicating a potentially deadly problem with gas embolisms (Gas Bubble Syndrome).
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.