Methylene blue can be used as a dip by itself and as an additive to a freshwater dip. As a dip on its own, it should never be used for more than 5 to 10 seconds MAX. It is most effective in this manner for fungal infections. Kordon's Methylene blue should be used in a saltwater dip prepared using 3 gallons of saltwater and 5 teaspoons of M.B. This produces a concentration of 50 parts per million. Note that these instructions are ONLY for the Kordon's Methylene Blue, as this is the bottle I have in front of me right now. When the dip is prepared, immerse the seahorse for just 5 to 10 seconds, then transfer him to the QT tank to rest.
Methylene Blue can also be added in small quantities to fresh water, but this is more of an anti-stressing regime than a disease-control tactic. The best article for it can be found
here, although I would adjust the amount of M.B. down a little because seahorses may be a little more finicky. Instead of 9 drops per gallon I would do 6 drops, but that's just my opinion. Hope this helps!