As you know, when administering antibiotics, the proper dosage for a marine aquarium is usually at least twice the recommended dosage for freshwater. In the case of neomycin, some seahorse keepers increase the dosage of neomycin sulfate up to four times the recommended dosage for saltwater tanks, or eight times the suggested dosage for freshwater (Keith Gentry et al.).
For example, standard protocol for treating seahorses for suspected bacterial infections with neomycin in a hospital tank is 250 mg/gal (66 mg/L) as the initial dose and 50% replacement with a 50% water change daily for a period of 10 days. However, if you are using the neomycin prophylactically to help prevent secondary infections, than the usual "double the freshwater dose" rule should suffice and you can administer the medication at the usual dosage recommended on the instructions for marine aquaria.
If you will be treating the hospital tank with neomycin, Seagazer, then I suggest administering the Diamox orally, which I find is often the most effective way to get the medication into your seahorse’s system, as discussed below.
Administering Acetazolamide/Diamox Orally
I have found that the Diamox is often more effective when it’s ingested and administering the medication orally may allow you to treat the seahorse in the main tank where he’s most comfortable and relaxed.
If you can obtain a small syringe with a fine needle, the acetazolamide solution can simply be injected into feeder shrimp or even frozen Mysis. Mic Payne (Seahorse Sanctuary) used this method of administering Diamox successfully when he had recurring problems with GBD due to maintaining a population of Hippocampus subelongatus in shallow tanks only 16-inches (40 cm) deep:
"Seahorses maintained in this system are susceptible to gas bubble disease. Specimens with bubbles around the eyes or under the epidermis of the tail are readily treated with acetazolamide (Diamox tablets 250 mg). Mix a very small amount of crushed tablet with water and inject it into several glass shrimp that are then frozen. These are then fed to the target animal at the rate of two per day for four days. Bubbles disappear on the second day."
Volcano shrimp or red feeder shrimp from Ocean Rider (iron horse feed) work great for this. If a fine enough needle is used, they will survive a short while after being injected — long enough for their twitching and leg movements to attract the interest of the seahorse and trigger a feeding response.
Leslie Leddo has cured seahorses with tail bubbles and pouch gas using this technique. She found that a 1/2 cc insulin syringe with a 26-gauge needle was ideal for injecting frozen Mysis or live red feeder shrimp. They plump up when injected and ~1/2 cc is about the most of the solution they can hold. There bodies will actually swell slightly as they are slowly injected and excess solution may start to leak out. The 26-gauge needle is fine enough that it does not kill the feeder shrimp outright; they survive long enough for the kicking of their legs and twitching to assure that they will be eaten. So if your Vet or family doctor will prescribe the Diamox for treating your seahorse, ask them also to provide a 1/2 cc insulin syringe with a 26-gauge needle.
If you are using 250-mg tablets, Leslie found that 1/8 of a tablet provides enough Diamox for several days’ worth of injections. In other words, 1/8 of a 250-mg Diamox tablet provides enough of the medication to inject two shrimp daily for about 5 days. So each day, I would take 1/8 of a tablet and shave off approximately 20%-25% of it to make the Diamox solution for that day’s injections. (NOTE: if you are using 125-mg Diamox tablets, adjust your dosage accordingly — that is, start with 1/4 of a tablet and then shave off 20%-25% of it to make the Diamox solution.) Then crush the Diamox you have shaved off and to a very fine powder and dissolve it in a very small quantity of water.
Use the resulting solution to inject two of the live feeder shrimp and feed them to the affected seahorse immediately after injecting them. You don’t want the healthy seahorses to ingest the medicated shrimp, so target feed them to the affected seahorse only.
Diamox doesn’t dissolve especially well in water; there’s always a residue of undissolved material left behind. Try to avoid this residue when you draw up the medicated solution in your syringe, the particles can sometimes clog up the fine bore needle when you are trying to inject the shrimp.
Each day you will have to prepare fresh Diamox solution to inject the shrimp for that day’s treatment, so just repeat the steps above each day. He should show improvement rapidly, with 2-3 days. If not, after you have fed him injected shrimp for 3 straight days, give him a break from the Diamox for a few days and try again. (Diamox can suppress the appepitite, so feed him unmedicated/uninjected shrimp for a few days to keep him eating and help restore his appetite.) Then feed him Diamox-injected shrimp again at the rate of 2 per day for a total of 3 more days, but this time increase the dosage of Diamox slightly (shave off a bit more of the tablet each day when you mix the new Diamox solution).
Best of luck resolving those early symptoms of GBS, Seagazer! The prognosis is always much better if you can nip it in the bud at the first sign of the condition, as you are doing.