Excess mucus production and scratching can be an indication of irritation from exposure to high ammonia levels, which can happen during long-distance shipping, are they can be a sign of external parasites.
For starters, I would suggest treating the irritated SunFire with methylene blue, which is effective against protozoan parasites and fungus, and is the treatment of choice for ammonia poisoning and nitrite toxicity.
Commonly known as "meth blue" or simply "blue," this is a wonderful medication for reversing the toxic effects of ammonia and nitrite poisoning (commonly known as "new tank syndrome"). Methylene blue also transports oxygen and aids breathing. It facilitates oxygen transport, helping fish breathe more easily by converting methemoglobin to hemoglobin — the normal oxygen carrying component of fish blood, thus allowing more oxygen to be carried through the bloodstream. This makes it very useful for treating gill infections, low oxygen levels, or anytime your seahorses are breathing rapidly and experiencing respiratory distress. It is the drug of choice for treating hypoxic emergencies of any kind with your fish.
In addition, methylene blue treats fungus and some bacteria and protozoans. Methylene blue is effective in preventing fungal infections, and it has antiprotozoal and antibacterial properties as well, by virtue of its ability to bind with cytoplasmic structures within the cell and interfere with oxidation-reduction processes. However, be aware that it is not safe to combine methylene blue with some antibiotics, so check your medication labels closely for any possible problems before doing so.
Methylene blue will destroy nitrifying bacteria so it should only be used in a hospital tank (if used in an established aquarium, it will impair the biological filtration and the tank may need to be cycled all over again).
If you can obtain the Kordon brand of Methylene Blue (available at most well-stocked local fish stores), their suggested treatment protocol for nitrite poisoning is as follows:
As an aid in reversal of nitrite (NO2-) or cyanide (CN-) poisoning of marine and freshwater aquarium fishes:
(a) Remove carbon filter and continue to operate with mechanical filter media throughout the treatment period.
(b) Add 1 teaspoon of 2.303% Methylene Blue per 10 gallons of water. This produces a concentration of 3 ppm. Continue the treatment for 3 to 5 days.
(c) Make a water change as noted and replace the filter carbon at the conclusion of the treatment.
See the following link for more information on treating with Kordon’s Methylene Blue:
Click here: KPD-28 Methylene Blue
I would like to see how your SunFire response to the methylene blue before we resort to any more aggressive treatments, Nick. Let’s see if the methylene blue eases his irritation and puts an end to that scratching, and see how the white stuff on the top of his head responds to the methylene blue. If the fuzzy white patch is not just excess mucus sloughing off, and/or the scratching persists, then it would be appropriate to progress to formalin baths combined with antibiotic therapy.
Best of luck with your new seahorses, Nick!