My approx. 13'' tire-track eel seems to be dying. Water parameters are normal: ammonia 0, water temperature 28 C, nitrate less than 2.5 ppm. I do 40% water changes every two weeks. I realised that something was wrong during the weekly water change which I've just finished. The eel is not hiding in its usual spot and seems to have difficulty swimming. Often, it lies on its back or its side for several minutes, then uprights itself and swims a bit, then the same thing happens. Sometimes it swims normally, sometimes it stays motionless. Help???