You're missing one. Should be Nitrite, rate, Ammonia, PH..Your Nitrite is good, but it should be baby blue, the top one. The Ammonia is cool. The PH should be 8.2(not shown). Nitrates are hard to get where they need to be. Not sure of the ppm, but is should be a yellowish color. Although, mine gets orange. Sometimes really orange. Yours look fine though. Get some SuperBuffer and add to your tank.
Also, an eel can live in almost anything. I've never known one to die, unless it got out.