Steve; The selcon is a good idea, perhaps also vita-chem. Of course any types of natural algaes you could get it to eat would be of help.
Its also possible that its larger size and limited space of a 55 gal. is causing stress on the tang. Just a thought, as they love open spaces.
BUT, here is a experience I had a few years ago with a sailfin. Just out of the blue, it developed LLE, like yours. From some articles I remember reading, I thought of the carbon also, as everything else seemed fine, including its diet.
I talked by phone with a few experts out there and one even had the same results as myself. I had just switched carbon brands to another well known product. It was my belief that the extra dust from some carbons and not the carbon removing anything from the water itself, was the cause. One of the aquarists I spoke to had the same results with the same brand
I wont mention the brand name, but can tell you whats good, that I have used. Marinelands Black Diamond is the best I have used and Two Little Fishies product is close behind, if not as good. There may be others, I am sure, but these two are the ones I have used and tested.
Anyways, when I switched back to Black Diamond, the tangs erosion reversed itself. Many aquarists run carbon on a regular basis and have healthy tangs, so I would say to make sure one does not use a product, esp. a cheaper one, that leaches carbon dust.
Of course, these are just my experience,s, and perhaps something else unknown came into play, with the sailfins erosion, but one never knows,
Hope that info is of some assistance to you.
I, too, haven't had any direct experience with this, but most of what I've heard and read is succinctly covered here: 
