Congratulations on successfully dipping your seahorses for the first time. Now that you know how to proceed, any subsequent dips should go smoothly and be much less intimidating for you.
When seahorses are suffering from external parasites, dipping them as we discussed previously is an excellent first aid procedure to start with. The seahorses may be temporarily stressed out from being handled and dipped in the freshwater, formalin, H2O2, or methylyne blue, but they should quickly recover once they are returned to the hospital tank or the main tank. Once they have had a chance to recover from the dips, the seahorses normally feel much better because many of the parasites will have been killed or dropped off of them while they are in the dipping container. They should therefore feel more like their old selves again, becoming more active and swimming and eating normally again. But the beneficial effects may only be transitory if the seahorses are returned to an aquarium with a heavy infestation of parasites and pests. Allow me to elaborate.
The type of dips we have been discussing will provide some short-term, immediate relief from external parasites for each easy horses, but the ponies are likely to become reinfested once they are returned to the main tank. The fact that your seahorses have resumed scratching themselves with their tails suggests that that is the case with your tank, mermaid, and they may also be reacting to the irritation from hydroid stings due to the hydromedusae microjellies. That’s why I suggested giving them a freshwater death and then transferring them into a hospital tank filled with freshly mixed saltwater so you could treat them with the Nitrofuracin Green.
That’s still what I would recommend. Set up a makeshift hospital tank with newly mixed saltwater as described below, give the seahorses another quick dip to cleanse them of some of the external parasites (dipping them in either freshwater, 3% hydrogen peroxide, methylyne blue, or formalin — whatever works best for you and you are most comfortable with — would do nicely for this step), and then transfer them to the hospital tank and treat them with the complete regimen of the Nitrofuracin Green, which works very well for treating mild skin infections. I am thinking that’s your best option for treating the seahorse with the "flakies" and your other seahorses should also benefit from the medication.
Setting up the Basic Hospital Tank
A bare-bottomed aquarium with plenty of hitching posts will suffice for a hospital ward or Quarantine Tank (QT). Ideally, the hospital tank should have one or more foam filters for biofiltration along with a small external filter, which can easily be removed from the tank during treatment but which can hold activated carbon or polyfilter pads when it’s time to pull the meds out. It’s important for the hospital ward to include enough hitching posts so that the seahorse won’t feel vulnerable or exposed during treatment. Aquarium safe, inert plastic plants or homemade hitching posts fashioned from polypropylene rope or twine that has been unraveled and anchored at one end are excellent for a hospital tank. No aquarium reflector is necessary. Ambient room light will suffice. (Bright lights can breakdown and inactivate certain medications and seahorses are more comfortable and feel more secure under relatively dim lighting.)
So just a bare tank with hitching posts is all you need for your hospital ward. No heater. No reflector. No lights. No substrate. You can even do without the sponge filters or external filter in your case, mermaid, just adding a couple of airstones to provide surface agitation and oxygenation. That’s it.
In a pinch, a clean 5-gallon plastic bucket (new and unused, NOT an old scrub bucket!) can serve as a makeshift hospital tank. It should be aerated and equipped with hitching posts and perhaps a heater, but nothing else. This makes a useful substitute when the Quarantine Tank is occupied or in use and a seahorse needs treatment.
Stay on top of water quality in the hospital tank/bucket with water changes as often as needed during treatment, and and when you are treating the occupants for a health problem, re-dose with the medication(s) according to directions after each water change
Let me know if your seahorses continue to scratch themselves when they have been transferred to the freshly mixed saltwater in your hospital tank and are undergoing treatment with the Nitrofuracin Green, mermaid, and I will explain how to administer hyposalinity to their treatment tank to eliminate the parasites that are apparently bothering.
Best of luck treating your itchy seahorses and reading your six-gallon nano tank of the mobile medusae micro jellyfish, mermaid.