Am I the only one who hasn't mentioned the obvious? Your fish load. I'm no expert on JDs but isn't 8 really crowding such a tank? I also thought that a 55 gallon could hold only 1-2 JDs at max.
I believe your fish load is too high, thus causing a huge imbalance in your tank... especially in terms of nitrates (and ammonia if your biofiltration isn't good enough). Check the ammonia, nitrite and nitrate levels of your tank.
-Richer