My fire eel (Mastacembelus erythrotaenia) has suddenly stopped eating. She (he?) is about 30" long, 6 years old, in a 210 gal tank [ammonia=0, nitrite=0, nitrate high, but it's always been high, 77F, pH about 7.8]. No visible lesions. Normal activity - not much - hangs out in one of her tubes...