Okay, I got the pictures and the mark on the back of your seahorse’s head is very obvious. But when I tried to enlarge the pictures to get a closer look they quickly became pixelated and I couldn’t make out any detail. I couldn’t tell if there is any scarring or lesion where the white marking is or whether it’s fuzzy and upraised or smooth and flush with the rest of the skin or sunken in.
As near as I can tell, the white marking on the back of your SunFire’s head appears to be a natural marking that has become irritated for one reason or another. Your seahorse has beautiful markings with a series of regular white blotches and saddles along its back and tail, and the white marking on its head appears to be a part of the SunFire’s natural color pattern, which has developed a problem. There could be a bit of an ammonia burn there or it could be the site of a localized infection (a bacterial or fungal lesion) or the irritation could be from ectoparasites, with excess mucus sloughing off as a result. The pictures aren’t clear enough to rule any of those out.
I think treatment with methylene blue is still your best option, Nick. Methylene blue should be available at most any local fish store, so I’m sure you won’t have any trouble finding it. If you can’t get the Kordon brand of methylene blue, just pick up whatever brand is available at your LFS and follow the instructions on the label or package.
If the methylene blue does not resolve this problem, we will try a regimen of antibiotic therapy in conjunction with a series of formalin baths, so for a backup medication you may want to pick up some formalin and some antibiotics when you are getting the methylene blue. Even if you don’t need them for your SunFire right now, those are good medications for a seahorse keeper to have on hand in your fish room medicine cabinet.
The antibiotics that are most useful for the seahorse keeper are kanamycin, nifurpirinol (brand name Furanase) and neomycin sulfate. They can all be used together at the same time to even greater affect, so it’s a good idea to to pick up all of them if you can. If they’re not available at your LFS, you can find them all online.
If the irritation and scratching persists and we determine external parasites are the likely cause, then it may be time to consider treating your main tank with hyposalinity, but given the soft corals and invertebrates you have, it would need to be a modified form of Osmotic Shock Therapy (OST). I will provide you with complete instructions if and when the time comes to consider that treatment alternative, Nick.
In the meantime, work on getting your ammonia and nitrite levels back down to zero and begin treating your SunFire with methylene blue as soon as possible. Please keep me updated and let me know if the methylene blue is having the desired effect or not.
Best of luck with your new seahorses, Nick!