In those three years you had the eel, is that all you feed your sfe? For the most part, one needs to feed many variety types of foods and not just one or two kinds and to feed twice a week, I would say that this is part of your problem besides the other questions I have in what your doing and the tank size and what type of skimmer you have.
Also, you need to add to its diet, crab and other types of foods like shrimps and such and feed twice a week until it refuses. Also, there be eels that you can feed once a week, but a sfe isn't one of those. Also, when you got this eel, what size was it? And I should had added at the first post on what if your any nitrite levels as well.
So we need to upgrade the eels diet some and too twice a week and use a feeding stick of some type and being that this eel could be ill looking, it just might drop the foods you offer behind the LR.
Also, tell the tank size and type of skimmer and how much LR it has and just in case, other inhabitants. Besides this, the power heads you are using for the tanks water currents. The eels main diet is crustaceans, but you can as well add squid but be sure its not too soft for you need to try to add as many variety types of foods to its diet and please do feed twice a week, later try to add small pieces of fish as well but do try to get some crab and be sure its healthy for the eel and you can do this by web searching for fresh sea food markets in your area.
And I just thought of this, are you doing water changes often? And how many gals? It may not be too late for you too save this eel, you just need to tell me everything as it really is and you can manage this to save the poor eel.
Good Luck