If the seahorse is only eating live ghost shrimp, then you should gutload the ghost shrimp with plenty of the medication (kanamycin sulfate in this case) before you feed the ghost shrimp to the seahorses.
There are couple of different ways that you can try gutloading the live ghost shrimp with the kanamycin. For example, you can soak flake food in a concentrated solution of kanamycin and then feed the medicated flake food to the ghost shrimp until the bellies of the shrimp are full.
Or another approach would be to gutload live adult brine shrimp with kanamycin and then feed the medicated adult brine shrimp to your live ghost shrimp before you feed the ghost shrimp to the seahorses.
Just add a generous amount of the kanamycin sulfate to one gallon of water and mix thoroughly. Place the amount of live adult brine shrimp you wish to medicate in the mixture and leave them there for at least 2 hours (the longer, the better). Then remove the medicated adult brine shrimp from the mixture and feed them to your live ghost shrimp and then immediately feed the gutloaded ghost shrimp to your seahorses.
In my experience, the best way to gutload the adult brine shrimp is to set up a clean plastic pail with one gallon of freshly mixed saltwater, add plenty of the antibiotic, and then let the adult brine shrimp soak in the medicated bucket for at least two hours before you feed the brine shrimp to the live ghost shrimp and then feed the ghost shrimp directly to your seahorses. Repeat this procedure twice a day for 10 days.
Hopefully one of these methods for gutloading live adult ghost shrimp with medication will work well for you.
Pete Giwojna, Ocean Rider Tech Support