Good question.
I find most of the time treating fish is a waste of time as the success rate is not that high.
From my disease book.
The seahorses main disease might be Costia a parastic skim infection followed by secondary bactrial infection.
Fish exhibit the following symptoms,
dark colouration,increased mucus buildup,turbidity of the skin, loss of scales,reduced appetite and fin rot,
treatment is with formalddehyde/malachite green or formaldehyde/copper.