It has been my experience that Hippocampus need not be considered delicate when it comes to disease treatments. Seahorses usually tolerate all the usual chemothberapeutics well, including copper sulfate, formalin, malachite green (or combinations thereof), and methylene blue. They typically handle antiparasitic agents (e.g., metronidazole, fenbendazole, praziquantel, and dylox) with no problems and do well at both hyposalinity and hypersalinity when it comes to osmotic shock therapy (OST). They can be treated readily with most antibiotics regardless of whether they are injected, ingested or administered as baths.
In short, I have found that Hippocampus generally tolerates the same treatments and standard dosages used for other finfish and marine teleost fishes.
About the only medication I specifically avoid when treating seahorses is copper sulfate. Although they tolerate copper sulfate well, it has the unfortunate side effect of suppressing the immune system as well as the usual difficulty of maintaining the proper therapeutic dose, and there are now safer medications and treatment options that are more effective than copper sulfate for treating Cryptocaryon and other parasites.
Not all medications designed for use in freshwater will work in a marine aquarium, Chris. This is because of poor solubility in saltwater and the fact that some of the medications may react with the salts and mineral components in the saltwater, or are not effective at the alkaline pH (8.0-8.4) we maintain in marine aquaria. For these reasons, many medications include different recommended dosages for freshwater and saltwater use, with the saltwater dosage usually being at least twice as much as the freshwater dose. In short, you cannot assume that a medication designed specifically for freshwater fish will be effective in a marine aquarium or that it will even dissolve in saltwater.
For more information on commonly used medications for seahorses that are known to be safe and effective, see the earlier discussion thread on this forum titled "Medications to keep on hand":
Best wishes with all your fishes, Chris! Here’s hoping they never need treatment with any medications.