ok.. some of what you said is really making me wonder what is going on in that tank...

Skimmer is not producing... why? even w/ a dirty pump it should still be producing something right? unless it's a needlewheel and something has hampered it from chopping up bubbles...

eels are scaleless and absorb o2 through their skin if i remember right... now, I used to have an african clawed frog that absorbed o2 through it's skin and it was sensetive to EVERYTHING.... i would have thought the eel would be the first to go, or if water conditions had gone that far south that the tangs would have come up w/ ich or something...

it really is wierd.

Get Bstreeps HIATT on the tank as soon as you can, that thing is voracious when it comes to eating waste.