How much DARK does coral need?


New member
Dark / Light = It's the flip side of the same coin...

Recently after adding more light, I cut back from a 9 hour day, to two 4 hour periods of lighting (4 on / 8 off) as I read some interesting things from coral propagators.

Initial results looked good, with generally a bit more robust corals.

I think if I kept it up it would in-fact speed up the growth rate of the corals.

Most people who undertook this 4 on/8 off approach said the fish didn't care.

None of my 'normal' fish did care... but my leopard Wrasses that bury themselves at night, and come out for 2/3rds the day before diving back into the safe sand were not seen for multiple days.

These fish seem much more attuned to the 'light cycle' and use it for their own purposes (when to go to sleep/ awake)... and the 4/8 thing really through them for a loop! The male hasn't eaten in a few days because he didn't know when to come out to find light!

I didn't want to risk the leapard wrasses time-cycle being thrown off too much, so I went back to a more standard 7.5 hours of light straight.

But here is my question: How much Dark does the coral actually need for good growth?

If my tank's lights are on from 3:30-11pm, and I give them a full 8 hours dark, would it benefit the corals at all to give them an extra 2-3 hours of light in the early morning (say 7-9:30am) and then 6 hours dark before the 'normal day' cycle of 7.5 hours?

This way the day-cycle-sensitive fish would still have their 'day', and the corals could still get a little extra "oomph".

I know those extra 2 hours wouldn't do much added on to the 7.5 hour day cycle (in my case of strong lighting, actually photo-inhibition is more likely)... but IS 6 HOURS DARK ENOUGH TIME for the coral to actually use that extra 'morning light', or will it just throw off the normal 'day cycle' and lead to photo-inhibition / interrupting the photo-periods and growth?

IE - is this good or bad to attempt... or Should I just stick to the day only cycle w/ 16.5 hours of dark?

Day cycle - 3:30pm-11pm
Dark cycle- 11pm-7am
small light cycle - 7-9:30
small dark cycle - 9:30-3:30

I KNOW the coral won't be as productive with only 2.5 hours of light (as they peak photosynthetic activity @ 3.5 - 4 hours into light cycle), but my question, is will the extra light HARM/interfere with the normal photo-periods/growth of the coral... or will it just give them a little extra photosynthetic potential?


In Memoriam
I found the dark cycle not all that critical to sps growth. my little sps nubs made a complete table top tabular growth form over 22 mos not actually having any real dark phase.

at night, I had a refugium in the back of the tank that was couterlit to the front dt. it would constantly partially light up the front tank all night long, no harm.


New member
Well I finally got around to looking more into this question...

spent a good 4 hours on Google Scholar today - much to the chagrin of my lady...

Not that many seem to care, but I figured I'd share my findings thus far...

There was not much if any actual research into this question ("How much DARK do corals need?"); but...

I've come to hypothesize that corals don't need all that much dark to 'reset' their cycle...

One article I read said that after exposing coral to 3 days of constant 'dim light', only 2 hours of dark time started to 'reset' the typical light/dark Motility rates...

Karin A. Lerch and Clayton B. Cook

Another paper led me to believe that 5 hours light is the 'peak' production/growth for coral (judged by Carbon14 increase in crushed samples...) 6 hours had slightly diminished activity. SEE GRAPH BELOW



This fits in with those who said corals can get by on 4 hours of light (the 4/8 light/dark thing I was trying)... 5 seems a little better...

Interesting enough, although photo-inhibition might inhibit photosynthetic production, within reason photo-inhibition was only negligible to the net energy loss to repair the tissue damage. (so although the coral didn't do as well with light over that 5ish hours, the coral can easily repair tissue damage from reasonable amounts of light in the more typical 8+ hour 'days')

"Energetic cost of photoinhibition in corals"
Mia O. Hoogenboom*, Kenneth R. N. Anthony, Sean R. Connolly


I might just try a slightly asymmetrical lighting rhythm... something like

5.5 on (day for leopard wrasses)
7 off
4.5 on
7 off

Another scientific paper I read before (don't have title/link) suggested that the presence of blue light (and ancient blue-sensitive zoanth) act as the 'activation' for the chemical /photosynthetic processes to begin (which we know peaks around 4-5 hours later...)

This being the case, and as I supplement actinic/blue light for a period before 'day' lighting kicks on, and with the dark periods a steady 7 hours each, I hypothesize that the corals should be able to acclimate + grow with increased potential, while giving my Leopard wrasse his more customary lengthier 'time above ground' (he was only out for about 6 hours a day before anyway...)
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New member
hard to comment on that pic without link to paper.
does not it mean that after 4h of light it is still only 1/3 of peak energy absorption , at 5th hour it reaches peak, 6th hour 77% of peak - so u need to reach at least 5th hour and keep it on for few hours so it will store enough energy to thrive?


New member
i Have been thinking of a break period mid day for my anemones and corals, like 6 hours lights on, 90 min off then on for another 5 hours or so. I had done this inadvertently before not knowing it and my anemones were doing fantastic. My timer is on a six strip on the floor and i accidentally stepped on it or something turning a few of the clips off. It was a month at least before i found out about it as i was never home at that time.
So it didn't hurt anything anyways.


New member
Some more research points.

Ammonium uptake and assimilation by zooxanthellae (Symbiodinium sp.) cultured with an excess of nitrate was enhanced in light. Uptake was decreased by the same amount when zooxanthellae were incubated in darkness either after 6 h pretreatment in light, or at the end of the dark period of a 12 h light: 12 h dark cycle.
The respiration rate of the Caribbean reef coral Porites porites was shown to increase by a mean of 39% above the pre-illumination respiration rate when exposed for 3 h to light equivalent to that at 10 m depth on the reef. When exposed to a subsaturating irradiance of 140 μE m-2 s-1, the respiration rate increased successively in a curvilinear form to 58% greater than the preillumination respiration rate after 80 min.

So it seems corals get more than half their amounts of energy from even 80 minutes of light... And the coral shows the same benefit from 6 hours dark as it does 12 hours dark.

Here's another cool article I found in my recent dig.

Low light corals evolved to get less light, but can take a lot in quick... but then their all like "we can't take much more a'this Capt'n!" Seems obvious but always cool to see some stats...

• 4
Three mechanisms of photoadaptation or acclimation were observed in cave and overhang habitats: (1) a 20–50% thinner tissue layer and 40–60% thinner skeletal plates, maximizing light interception per unit mass; (2) a two- to threefold higher photosynthetic efficiency per unit biomass; and (3) low rates of dark respiration.
• 5
Specimens from open and cave habitats displayed a high capacity to acclimate to downshifts or upshifts in irradiance, respectively. However, specimens in caves displayed limited acclimation to further irradiance reduction, indicating that these live near their irradiance limit.


Judging from articles posted before, it seems like 2 hours dark does the trick to reset their circadian rhythm = a good nap as I take it; But you really don't want to do that until at least 5 hours into the light when your production has peaked.


As I take it, the corals absorb full production of light status ~5 hours in. After that, I would assume the irradiance begins to overwhlem the coral, and we see minor tissue damage. But the energy it takes to repair the damage is minimal at first (only small decreases in overall productivity) but continues with longer exposure to irradiance [light].

Ergo, 5 hours of light is all they need, 6 is Good; 7 is Pretty Good; 8 is Alright; 9 Ain't too bad... but you get the picture. Think photoinhibition. It's certainly not going to kill the coral (until you keep going...) but the coral is still losing more energy in cellular repair that it could otherwise spend in growth.

Keep in mind that the photosynthetic process is primed by the presence of blue light, and doesn't then the coral may receive more than half the energy it needs after 80 mins... but doesn't reach full potential until 3.5 to 5 hours into the light cycle...

And we have small window (that 1.5-5 hour mark) to 'get all we can and get out' [hit peak photosynthetic production]... and after that its a mostly negatable but none-the-less ever increasing net loss to the system.

And corals are just as photosynthetically productive after 6 hours dark as they are after 12 hours of dark.
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New member
upon re-reading the article by McAuley + Smith i think I misunderstood - they didn't say 6 hours of dark was same as 12... Happens after a few hours of articles. Pardon.