I think a good O2 test kit will resolve whether a tank is well oxygenated. I do believe strongly that my protein skimmer is the only reason my tank is at about 100% saturation (Other devices would work too but I don't use any other aerating devices).
Here's a couple of O2 saturation tables for salt water. My test kit shows a level of about 7 (+/-) at 25C.
http://ourworld.compuserve.com/homep...S/Oxygen-1.htm
http://www.emersonprocess.com/raihom...ers_200208.pdf
I do know that before I added the protein skimmer (when I started the hobby 1 year ago), I was losing fish like crazy from disease or stress over a week or 2 even though I was adding fish slowly (and replacements of the same fish would die too. Others like clownfish had no problems at all). At that time, the tank was agitated with only a powerhead at the surface and an internal fluval filter. There was a lot of water movement at the surface. I tested for Ammonia, Nitrites, Nitrates, Phosphates, etc and everything was near perfect. I couldn't figure it out.
Then, I installed a protein skimmer and then I noticed my replacement fish all started to survive. I thought ok, maybe its because my tank is more stable now or maybe it was just bad luck or maybe it was the ick that killed them before. Then, an event occurred one day that convinced me that my fish were dying from reasons indirectly or directly from lack of oxygen. One night I turned off the skimmer (so I can sleep better since the skimmer was noisy) and the next morning, my O2 loving fish was laying on its side gasping for air. It looked like it would die within minutes. I plugged the skimmer back on and within the hour, my fish was rejuvenated and is still alive with me today after 9 months. This does tell me that my tank is overstocked, and that some fish require higher levels of O2. But it also tells me that the skimmer does a better job of aerating my tank than the powerheads (w/o venturi) at my water surface. Nowadays, my fish still gets ick once in a while (after a stressful water change) but this time they survive and it goes away on its own.
This is my story and it might not be related to any of you who has lost fish but I do suggest getting an O2 test kit and just eliminate that as a cause. I wish that I did test the O2 level of my tank when my fish were dying so I could document what O2 levels are bad. It probably might have been useful to know which fish started dying at levels of 5 or 6.