Haha. I'm tempted to buy a Moorish Idol, put it in my tank, turn off the skimmer for about 2 hours during the day, take a video of the MI dying while all other fish are active, then turn the skimmer back on to see it recover just so I don't have to keep on about it. :) I've seen it 2 or 3 times before.

Sam, I'm a believer. However, that doesn't mean there aren't other factors. I'm hoping that with my "surf tank", I'll have no troubles. He's eating well and seems well settled, so we will see.

Yes, other factors goes without saying. But healthy fish all dying during the night (and the fish which I know would be the first to suffocate) while others have no problems. Also, all else constant except for the flow of oxygenated water back to the display tank.

On my tank, I also had surface ripples from powerheads and aquaclear running when the skimmer was off. Not enough.

Back to food. Looking back, in additional to zoas, mine also devoured: Hydnophora, Candy Cane, and Moon corals.

Maybe I'll look for some of these

Back to DO:

My own DO measurements support a lot of what was discussed here in this article: The Need to Breathe, Part 3: Real Tanks and Real Importance

It's a long article but I'll just briefly post a little bit of it here.

http://reefkeeping.com/issues/2005-08/eb/index.php

Tank 1: "Clownfish Tank"


Test 1: Circulated with powerheads and no skimmer: Morning DO saturation: 16%

Test 2: Aerated with protein skimmer: Lowest DO saturation: 81.2%

Same tank but aerated with airstone: Brought DO saturation up from 51% to 87% in 30 mins.


"
Tank 1 Discussion
The ten-gallon tank containing clownfish has been set up as an unskimmed system with what I consider to be an average stocking density of organisms for a tank of its size. I had assumed (wrongfully) that oxygen was maintained at high levels through the use of two powerheads that agitated the water's surface. However, once the lights went out and photosynthesis stopped, oxygen levels dropped quickly from a high of 78.7% of saturation to a hypoxic low of 16% of saturation. The levels were apparently low enough that each night, the clownfish would leave their anemone and adopt a position just under the water's surface directly above a powerhead. Out of concern, I then monitored the changes in oxygen levels at night using an airstone. Oxygen rose quickly and dramatically. At that point, I added a skimmer to the tank, with the result that oxygen is now maintained at much higher levels, ranging from a high of 130% of saturation to a low of 81.2% of saturation. However, it is only when the lights come on that oxygen reaches saturation or becomes supersaturated. It is notable that there appears to be a period early in the day when oxygen levels are maximal, with a depression to slightly subsaturated levels over the course of the afternoon. Also notable is a slight, but noticeable, drop in oxygen immediately after feeding. This measurement has been made repeatedly and is consistent.
"


"-Aquaria can and do become hypoxic at night and such a state may pose a risk to hypoxia-intolerant organisms.

-Aquaria can and do become saturated or supersaturated with oxygen during the day, and this is a result of oxygen resulting from irradiance of photosynthetic organisms. In no case was saturation or supersaturation measured without photosynthesis.

-Airstones and skimmers appear to be a very effective means of oxygenating small water volumes.

-Powerheads and recirculating pumps do not appear to greatly increase the oxygen saturation state of seawater aquaria.
"

So, thinks to himself, Apex must make a DO meter, right?? Ya, Google the price on that one!! lol

$700? Sell more frags instead of dumping them onto your lawn.

And another 100 for the module. US. So near a grand. I'll just keep the pumps pumping :) I'm years away from frags lol

Ok, just note that powerheads (nor external hang on back filters) really don't oxygenate the water much according to the article and my own tests and results. You really need to break the water surface with really violent splashes that would really make a mess or for ease, just use the skimmer/airstone

If there's a place to rent DO meters on the Island (In Burnaby, there's Hoskin Scientific) you could find out what your morning DO is with your pumps dialed down to its minimum and compare to when it is running at its max. Just take your angels and MI out of the system when you are testing for minimum levels and keep them in other oxgenated tanks (sump?). Although keep into consideration that the levels in the display tank would be a bit lower once those demanding fish are put back in.