Water flow for corals - reduce overnight?

[Barney121]

I have a 75G tank with 4x95W PC lighting, 100Lbs LR. Right now it's a coral grab bag (more than intended because of some shipping substitutions) with the gamut of red mushrooms, toadstool leather, caulestrea, purple gorgonian, acropora, fox, merulina, and hammer coral. Plus 2 clowns, firefish, blenny.

Circulation is provided by 2 x Maxijet 1200 (rated 295 GPH each), and a Magdrive 500GPH feeding a SCWD to provide alternating currents. Also there's Remora Pro skimmer and a power filter.

Here's the question. With both the powerheads and Magdrive running, and figuring about 60 gallons of actual water and using the full GPH ratings, I get about 18x/hour water turnover. Right now I have the Magdrive hooked up on my timer to run only when the lights are on (about 11 hours/day). The water movement is thus much less at night, maybe 10x/hour, and especially lower on one side of the tank. Would it be better to keep the powerheads and pump on all the time? Or to give the system some slower circulation overnight as I have it now?

Thanks in advance for your thoughts!

[Reefmedic79]

MOst wavemakers have a night setting which calm the current for 8-12 hours so I wouldn't see any problems with it. Is the Magdrive 5 being used as a return? I would set it up to have the PH"s go off with the lights instead of the Mag.

[Barney121]

Originally posted by fleetmarine79
MOst wavemakers have a night setting which calm the current for 8-12 hours so I wouldn't see any problems with it. Is the Magdrive 5 being used as a return? I would set it up to have the PH"s go off with the lights instead of the Mag.

The Magdrive is actually submersed, just feeding the SCWD which alternates 2 directions. It's certainly no problem to swap the powerheads with the magdrive, it's all just internal to the tank. I guess I was curious if having a period with lower flow is beneficial to the corals, or if the continued higher flow to keep water moving everywhere was better. Your comment about the other wavemakers does suggest a break is fine. Thanks.

[nasoman007]

Barney121,
I just bought a merulina and found out that they need high light, but i only have pc's like you. Is yours doing fine with pc's? How long have u had it?

[EricHugo]

Merulina do not need high light.

on the water flow, nightime currents in the wild are generally less except during storms, wind, and tide changes, but that is not necessarily a good thing for tanks. Tanks have very high animal loads and reducing water flow might make O2 drop more than it does, perhaps dangerously low in some cases. I would worry about waking up to see dead fish in a heavily stocked 75 with only two maxijet 1200's in there. I would run the Mag constantly, and up the circulation considerably in any case.

[Barney121]

Originally posted by EricHugo
<snip>

Tanks have very high animal loads and reducing water flow might make O2 drop more than it does, perhaps dangerously low in some cases. I would worry about waking up to see dead fish in a heavily stocked 75 with only two maxijet 1200's in there. I would run the Mag constantly, and up the circulation considerably in any case.

Thanks for the thoughts (and your AC book, and your participation here). Although it hasn't been heavily stocked, I haven't had the fish problem you mention. But also, I guess I'm trying to understand some of the (misleading?) rules of thumb here. Paletta (a few years old) suggests <10x per hour water circulation. The highest I've ever seen routinely suggested for reef tanks is about 20x. The 2 maxijet 1200x on paper give about 10x circulation (~600 GPH/~60 G H2O), and adding the other pump gives (on paper) close to 20x. Would your thought be that I'm overestimating real flow (possible given reality vs. ratings) or that the 20x rule of thumb is too low?

Given what I have, though, it sounds like the answer to "will ~20x circulation 24 hrs/day be detrimental to fish or corals?" is "no," so I'll swap around my circuits and keep them running. Thanks again!

[Also, nasoman007, my Merulina is doing great, although it's only 6-8 weeks so far, about 12" down under the 4x95 PCs.]

[EricHugo]

Tank turnover time is about as useless as watts per gallon. What happens if you have a 4" return outflow? Water velocity will be very low even if turnover is high and the tank will be functionally stagnant at the surface of corals. Oxygenation happens largely through surface turbulence, S:V ratio, and water overflows. If it is a tank without an overflow (HOB skimmer) and powerheads way below the surface and a 55 gallon style "narrow" tank, then you could have a lot of functional water flow and still have low O2. With a 4' x 4' x 1' tank, the water could be near saturation with O2 and have very little flow save maybe a small powerhead pointed at the surface, but the corals might not do well from having low flow at their surface. Rock formations and corals block flow, and so multiple outflows are usually better than flows from one or two points. Plus, not all species have the same flow requirements.

So, its a tricky question, and no one right answer. Certainly not 10-20x turnover.