So let me ask this, because all of my corals and fish seem to be doing very well: how will the adverse condition of lack of flow at night manifest itself with respect to the appearance or lifespan of the animals? Would it be a gradual decline in health?
Slowed growth would be the biggest adverse effect. Flow is extremely important for calcification, which despite previous assumptions, has been shown to occur at a significant rate even in the dark. It's also important just for respiration and lowered rates of respiration would be expected to yield slower growth and reproduction.
The tunze provides low flow at night but it probably isn't substantial enough to cause death or decline just speculation, if anything it may very well be enough for the night micro creatures to be more motile and easier capture for the corals? But to know any answer what is to low and how accurate are the modern measuring techniques.
Actually, small, benthic critters tend to stay within the momentum boundary layer so they experience low flow speeds, even in high flow environments. Their movement isn't greatly restricted by high flow. Planktonic organisms are equally active regardless of the flow since they really have no choice. It's really just a matter of how fast the water moves them and what kind of pattern it moves them in. At low speeds they tend to move laminarly and at higher speeds they move more turbulently. The latter is more favorable to feeding. However, corals tend to make a trade off since high flow deforms their tentacles and makes them less suited to catching food. As a result they tend to have the highest feeding rates at moderate flow speeds. There has been a lot of work done in this area, but unfortunately I can't really look up most of it from the field here. However, I would suggest you look at some of Ken Sebens' work as well as some of Brian Helmuth's.
Also, there have been a lot of good numbers published for flow rates in nature. Some reasonable ranges are <5 cm/sec in lagoons, flats, and deep forereefs, 5-15 cm/sec on the backreef and shallow forereef, and 15-50 cm/sec on the reef crest. Unfortunately, since most reef tanks are a jumble of corals from different reef zones, it's really hard to say what the desirable or safe value would be.
There are some really low tech but proven methods for measuring these things though. One way is just to use dye, (usually fluorescein, but food coloring should work) and a meter stick (or a ruler for in tanks) and just measure how far the dye moves in a given time period. Another way is to measure the dissolution rate of a solid such as a block of gypsum, plaster of Paris, or even a wint-o-green lifesaver. There have been papers published on how to correlate the dissolution rates of all three to flow speeds. The second method is a little more involved, but it's also usually more accurate. I would check the chemistry forum first to make sure none of these things would cause problems if your dissolved them in a closed system though.
