I feel fairly bad about this, but it was still a somewhat enlighting experience, so I thought I would share it.
I have a 10 gallon... I will call it a quarantine tank (very loosely). Its sitting on my kitchen counter, it has a bunch of macro algae and 4 pinfish from the gulf of mexico along with a couple grass shrimp some mexican turbo snails and some hermit crabs. It has a small ~70-80 gallon powerhead and an aquaclear 70 (400 GPH). Assuming the aquaclear flow is reduced some lets call the whole turnover 400 GPH, thats 40x tank size. I have had this setup going several weeks now.
All the algae has been growing, the algae on the glass and the macro algae, I have been feeding fairly heavily (couple pinches / day) and I am sure the nitrates are going up a bit, but I wasnt worried about it too much. This morning I came out right after the lights came on and 2 of the fish were dead, ther other 2 were laying on macro algae gasping for breath, the snails and hermits seemed unaffected, they were climbing on the glass and doing their thing like normal, did not seem stressed in the least. There was no power flash or flicker last night, flow was going on all night same as normal.
So from this accidental experiment a couple observations:
Smallest and largest fish died from lack of O2, medium 2 survived. (smallest one looked like it had been dead a while, larger more recently)
Inverts dont seem to need as high an O2 concentration as verts (fish)
Macro algae can consume a LOT of oxygen when the lights go out.
It made me wonder about people with macro algae filled refugiums, and how much they deplete the O2 when the lights are off. I know there was a discussion here about algae screens dying when the power goes off, and poisoning the tank when power comes back on, but that effect would take a relatively long time, perhaps days. However, noticing this effect, even with TONS of flow in a 10 gallon, I wonder how much dies off in a macro algae filled refugium even during a relatively brief period of power outage, and how much it ends up polluting the display tank when the power comes back on. It made me glad that my algae isnt in the tank when my power goes off.