A while ago, not too long ago another member had lost his Zebra moray and it was all due too a combination of water aquality and a healthy diet. I understood from what I readied that his eel eaten mostly only shrimp. But nitrates were through the roof.
Being that your a reef eel as that other member, you need to best have your salinity up there, not in the lower safe zones, but rather 1.023-24 as the lowest. Strictly reef eels do far better with it, if in a FO tank.
As the other thing which I pointed out, that eels in general will suffer a great deal longer then any other fish a hobbyist will keep. The signs from the eel will be extremely slow, and before most hobbyists realize their eel is with a problem, most loose their eels.
I have too believe that your nitrites and ammonia are good, what of your nitrate levels and the Ph? Salinity?
Im guessing that the tank is a 55 gal tank, what else is in this tank and how much LR?
Please do list the diet you been feeding it. Vitamins?
If the eel is getting the best in water quality and in its diet, Vitamins wouldn't be necessary.
Buddy