I have been setting my Vortech20 to night mode as my actinics shut off at 9:00pm. Sleep mode ends 10 hours later at 7:00am.

During this time my 2 clowns are definitely in sleep mode and slowly start to perk up after the room lights come on and after the Vortech comes back to full setting. During their sleep time they are definitely using less oxygen due to less movement.

But I might try turning off the sleep mode for a few days and see if there is a difference in their response. I have very little in corals at the moment to judge their response.

That is a fallacy. The actual movemement that is created by tiny bubbles around the area of the skimmer is far less than the entire footprint of your tank. If you look at the top of your tank, every micro-ripple is exchanging gases, CO2 for O2. Each of those ripples is supersaturated across the footprint of the top of your tank. This also includes your sump. BUT, the footprint of a skimmer, and its immediate surrounding area is vastly smaller than the rest of the tank.

Why don't you do an experiment with a heavily stocked tank. Take the skimmer offline, and keep the wavemaker in full blast mode. Then do the reverse.... I bet you that some of the corals could be affected by the latter method and not the former.

Interesting argument. My tank has a refugium filled with Cheato on a reverse day-time light cycle, it also has a large overflow and a 50 gallon sump, it is a lay down 135. I have a return pump that also powers my skimmer running at 2200 gph and my powerhead does 2000 gph. In my tank I feel that having my powerhead turn down to 1000 gph at night does not significantly reduce the 02 in my tank due to surface gas exchange, but does give my fish a respite from the current and a chance to get some good R and R. So I use my nighttime feature on my profilux / tunze combo, but I have no data to support my argument.

The trouble with that statement though is that you are looking at the skimmer and sump drop foot prints as 1 dimensional when in fact the water in the skimmer or falling into the sump is surrounded by bubbles. I would be willing to bet that if someone much smarter then me figured the amount of "surface area" the water in the skimmer has we would all be floored.

As anecdotal evidence, I can point to the common practice of basement tank owners hooking their skimmer air intake up to a fresh air pipe and watching that fresh air alter the Ph of the tank. Also I would point to the method in Fresh water of CO2 injection via a bubbler again, changing the Ph through the gas exchange between the bubbles.

Not discounting the gas exchange that occurs on the surface of the water, I just don't see how that could be greater then what occurs in the skimmer.

FWIW, when my pumps go into night mode I keep the same patterns and just drop the flow by 20-25%

All IMHO

It wouldn't be too hard to get some idea of the oxygen levels in a tank over a period of time. Anyone with an ORP meter would get a good (if not exact) indication of this. All such as person would have to do is monitor ORP levels at night with:
1. the skimmer on and pumps on full
2. the skimmer on and pumps on night mode
3. the skimmer off and pumps on full
4. the skimmer off and pumps on night mode

Then compare the results with daylight readings.

I read an article on this very topic some time ago. It was more to do with the resperation of the coral vrs the exchange of gas. It gave a very good example of how it works. Most creatures have specialised breathing such as our internal lungs and fish gills. Coral however have external polyps that intake the oxygen they need. Basicly it said it would be akin to having our lungs external and getting the flow of air brought in by the wind. On calm days you would find yourself wishing for gusts of air to fill yours lungs wile a blustery day keep them filled.

I will try and find it and post it here.

Thanks for everyones input, very interesting with some good points. I'm mostly concerned with what will benefit my corals most. Zerandise, I hope you can find a link to that article you mentioned.

This was discussed on RC recently. I believe, if I recall correctly, it was determined that the commonly held misconception in this hobby that the current on a reef is lower at night is, in fact, incorrect.

Do a search on RC for "night" and I'm sure you'll find the thread.

Read the second to last post in this thread:

http://reefcentral.com/forums/showth...ht+AND+current

Here is a link to Part 1 of the article. Its 5 parts in total.

http://www.advancedaquarist.com/2006...ew?searchterm=

"We take for granted that we have very specialized breathing structures: our lungs are internalized and actively create favorable concentration gradients by forcing air into narrow passage ways so that we can breathe properly. Fish have external gills, crustaceans have internal gills, insects have trachae and even nudibranchs have external branchae. All of these structures feature a very high surface area and a good deal of vascularization. Corals, on the other hand, have no specialized respiratory structures. Their external anatomy only features tentacles, a mouth, some tissue in between the polyps and, in the case of soft corals, they also have some pinnules along the sides of the tentacles. None of these are differentiated into specialized respiratory structures even though they have to rely on these anatomical features for gas exchange. If you had to breathe (respire) as a coral breathes,this would be the equivalent of holding your lungs outside of your body, inside out, and just hoping that the wind would blow hard and long enough for you to be able to breathe. This is how corals breathe in their environment and the scenario above illustrates the importance of water flow for adequate gas exchange in corals. This perspective might make you think twice about reducing your aquarium's flow at night."