By "12 gal" I assume you have a NC12, which really only holds about 9 gals of water, which is way too small for larger species SH. It is most likely running way too warm as well (most run right around 80*F), which can result in bacterial infection more often than not.
What are your water parameters...HN3, SG, pH, NO3, temp...("fine" isn't the right answer)? How old is the tank?
First of all, you need to get your tank down to about 68*F or less to help arrest any possible bacterial involvement.
Is the SH a male or female? If it's a male, is the pouch swollen? If this is the case, you'll need to perform a pouch evacuation. I can tell you how if need be.
If the SH is a female, or a male without PE (pouch embolism), you'll want to get some Acetazolamide (Diamox) and treat the SH for edema. This is a 3-day treatment, but works very well (again, I can tell you how, but Diamox is a prescription drug from a vet, so you'll need to get that). Many vets won't write scrips for fish because they don't have any dosage reference. Tell the vet that it's 250 mg per day per 10 gals for 3 days. However, you don't just pop the med into the water, you'll need instructions.
I'm guessing you got the SH from a LFS and it was fairly cheap ($25 - $30). This SH was likely pen-raised in the ocean and has been exposed to all of the same parasites as true WC specimens. These fish were actually raised for the TCM (traditional chinese medicine) trade, and most are dried and ground into powder, so "quality" isn't a real issue here.
As you're finding out, these "inexpensive" SH come with a "cost of ownership" that make them quite expensive in time and meds to get them up to snuff. They also give novice keepers the false impression that SH aren't possible to keep. Bad juju all around...
HTH
