Alternatively, lots of algae will contribute CO2 at night, which is when I lost my fish.

Yes, that's why for this test, one could turn off the lights to increase the depletion of O2 in the tank. See Eric's clownfish tank test where his DO dropped to 16% at night. And overflows are like waterfalls I guess so that is probably oxygenating the water. If that water is coming back into the display at large volumes, then the display tank is oxygenated no matter whether lights are on or off.

Even so...
it would seem you two are reporting extreme oxygen sensitivity.

Where do you draw the line for the threshold?

Now it sounds like the average set up with some algae, overflows and such would almost certainly be able to maintain appropriate oxygen levels.

No not extreme. Most other fish can tolerate these levels because it is not extreme. That's the point. Different fish have different tolerances to hypoxia.

I draw it closer than I would have last month. Otherwise "normal" setup, albeit pumps turned less than 30%, lots of algae, lots of dead (large) fish. MI first, then angels. Different evenings. Dog sitter would see fish at last visit, pick out a new body at 9am.

Right now, this is pretty thin guys.

Super interesting though and well worth exploring.

The fishes you have reported dying have extreme sensitivity.
not sure if you read that right (or I typed it right).

Yup. Agreed. However, I'm not willing to test this, just going to amp up the flow :)

How about we take this over here

http://www.canreef.com/vbulletin/sho...465#post936465

MI thread getting a bit off track.

Yup. that's what I said. The fish that are dying are extremely sensitive. But the conditions are not extreme. For example (i'm making these numbers up of course) most fish can survive under 50% DO saturation. Because most fish can survive at 50% saturation, this level is not considered extremely low. However MI and dwarf angels can not. So they are extremely sensitive. I made those numbers up to illustrate my point of course. I would have to measure the DO to know exactly at what point the MI starts to suffocate while all other fish do not. But it is probably at some level that most people do not think their tank is low on DO. DO fluctuates wildly depending on many factors. I've measured 40% before in my old system with airstone skimmer on with no deaths. My old system went as high as 130% during supersaturation by photosynthesis. With my current system, the lowest it'll get is 80% and the max is 98%. I'm sure that when I experienced the deaths overnight with aeration off, DO was well below 40%.

Sorry, you are right. The fish are extremely sensitive. I misread that the first time. The conditions do not need to be extreme.