Water Turbulance at Night: Throttle Down or Up?

Many reefers today strive towards mimicking mother nature. From trying to get those parameters to match natural saltwater, to aquascaping their tanks to perfection. Many are embraced to succeed in this hobby by doing it like nature.

The hot topic of late is water motion at night. Many reefers purchase controllers or devices to move water in multitude of ways. These controllers can throttle up or down your powerheads/wave devices to mimic surges or calm waters. The common belief these days on this topic is that you should throttle down your water current at night, to allow organisms to rest, according to nature. This is becoming a big misconception and people are getting themselves (and their tanks) in a big trap.

Take it from a marine biologics at Reef Central:

Because when the lights go out there is heavy respiration by all the animals and plants in the tank, but there is no photosynthesis to replenish the oxygen consumed. Oxygen levels, even in nature, drop dramatically and the water near the reef surface becomes hypoxic at night. Water motion helps to bring in oxygenated water and is especially important for animals like corals that rely on diffusion of O2 from the water column. There’s actually a thin layer of water that “sticks” to the surface of the corals and becomes especially low in O2 as compared to the overlying water. This boundary layer forms a barrier to diffusion of O2 in and CO2 out of the coral, essentially suffocating it. The faster the water flow, the thinner the boundary layer. It doesn’t make sense to increase the boundary layer when O2 is already at it’s most limiting.Essentially none of our animals actually sleep and those that enter some state or torpor don’t need lower flow to do so.

So .. no O2 at night? I think I will stick to higher gph at night, thank you very much. At a higher gph (gallons per hour), there will be higher water turbulence/agitation at the water surface and hence will add more oxygen into the tank.

I think that we need to be aware of this potential disaster. If you have a lot of livestock, and you throttle down your water motion (or surface agitation) at night, you are in ‘high risk of a tank crash’ category.

Chris on Reef Central added:

I’ll also point out that in the shallows that get a lot of circulation from wind-driven waves which may indeed calm down at night (but not as a rule by any means) the waterflow is typically much stronger than what is in anybody’s tank. The surge from even modest wave action at a few meters depth is so much stonger than what is in anybody’s tank. You can kick like crazy to maintain your position and it doesn’t matter–you still get tossed back and forth. I’ve yet to see any aquarium that had waterflow so strong that I could not swim against it.

So until we do get water motion devices that our fish cannot swim against, we should just stick to keeping our powerheads/water motion devices at a medium to full throttle at all times.