Mr. Saltwater Tank

Terrible Advice Tuesdays (T.A.Tues): Nuisance Algae Control Method Kills Corals?

Terrible Advice Tuesdays (T.A.Tues): 3 days of darkness will harm your corals

The rest of the story: 3 days of darkness is a method used to control nuisance algae. The idea behind it is simple: without light, algae dies. Also simple is how you perform 3 days of darkness: just turn your lights off for 3 days.

Once you turn the lights back on, you’ll be amazed to find that your corals are perfectly fine! Think about it: out on the reefs in the ocean is it a perfectly sunny day 365 days a year?

No. There are plenty of days when the sun doesn’t shine and sometimes doesn’t shine for days on end. Your corals can handle the 3 days of darkness no sweat. Of course if you have a coral that is dying then the 3 days of darkness might do it in and if it doesn’t survive 3 days of darkness, it probably wasn’t going to make it anyway.

And as a precaution I recommend you ramp up your photoperiod once you turn your lights back on. Probably not needed and it doesn’t hurt to be safe.

Finally, if you are relying solely on 3 days of darkness to fix your algae problem, here’s some bad news for you: the algae is going to come back sooner or later because you haven’t fixed the underlying issue on why you have an algae problem. 3 days of darkness can be part of the solution for fixing an algae outbreak, but it is not a lasting solution.


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Comments for this article (14)

  • carlson says:

    Just buy red slime remover. As long as you cut off your protein skimmer it works fine. After two doses algae no longer comes back in my tank.

  • Edgardo Cruz says:

    What also is a prenentative method is an elevated pH (8.3-8-4) and a magnesium level of 1500ppm+.

  • Edgardo Cruz says:

    What also is a preventative method is elevated pH (8.3-8-4) and a magnesium level of 1500ppm+.

  • Deep End says:

    Is “photoperiod” simply the time lights are on?

  • Deep End…correct

  • Luke says:

    Sounds like a good idea… but doesn’t make too much sense to me. Seems like there is algae on the reefs in the oceans as well… will 3 days of darkness really do any damage? I guess I’ll just have to try it myself! I’ve noticed my algae problems are seasonal. I get a fair amount of hair algae for about 3 months in the summer, then it dies off and all is well the rest of the year. What’s with that?


  • Micah says:

    Mark, why do you suggest ramping up your photoperiod after three days of darkness? Don’t you recommend that you acclimate new corals to lights slowly, especially if they were shipped? Wouldn’t it follow that you should also slowly acclimate corals after 3 days of no light?

  • Churn says:

    I tried 3 days of darkness, and my (which I though was) previously healthy clam died from that.

  • Churn…as I outlined in my midday monday Q&A show about clams, clams usually die a long slow death all while seeming totally healthy. If the clam was healthy and not dying, 3 days of darkness would not do it in

  • Zeeshan says:

    Hello All,

    I’m from U.A.E (aka Dubai) and have setup my tank 88G in April 2014. All seems to be fine, SPS corals and fishes seem to be happy. however i’m having this hair algae problem. which does not want to go away, additionally recently, new lush green color layer type of algae has started to form on glass. would anyone have an idea what this is?
    Nitrates: 0ppm (API testkit)
    Phosphate: 0.1ppm (Salifert testkit)
    weekly 20% water change.
    feeding 1 time a day: brine shrimp frozen food, i see no leftovers as fishes consume all of it.
    light: maxspect 160W
    total photoperiod: 9:00am-18:00pm (% of light intensity is @ 100% from 12:00 to 14:00).

    any advise/suggestion most welcome.


  • JasPR says:

    I’m sorry if this is preachy or if taken as a bit obnoxious ( it isn’t meant too), BUT treating symptoms is a useless exercise– I realize we live in a society and culture of instant gratification and extreme beliefs that there is some new ‘apple product’ that can fix anything. Still, 99% of problems encountered in our boxes of water are ecosystem driven and the solutions are to balance an imbalance and not to plaster over a symptom.
    Algae likes- warm water, water high in carbon dioxide and low in oxygen ( the warmer the water and more still the water, the higher the carbon dioxide and lower the dissolved oxygen). Algae likes FOOD- phosphates from fish food ( frozen is loaded as is pellets) and nitrates ( see maintenance and ‘old tank syndrome’). THEN and only then– algae has the necessary ingredients to LOVE light in the photosynthesis process.
    If you want to declare war on algae– FIRST appreciate that a box of water is deteriorating with time. And deterioration means plant food increases, pH drops and plants take over some of the nitrogenous waste processing. You can take away one factor or another but unless you make the environment ‘hostile’ for algae, it will always come back. Indeed it always lingers in the background as so mush ‘white noise’ waiting for conditions to favor its return. it needs to be discouraged and crowded out with other forms of microbes.
    The reefs of the world are actually deserts when it comes to nutrients. The nutrients that are there are living and the system nails every gram of organic material made available. in other words, it is always in ‘play’ and not building up. Our water boxes are the opposite, things are always building up and/or declining in other areas. Nuff said, JasPR

  • James says:

    Hi mark, I’m fairly new to the reef keeping hobby, been in it for about 6 months. I have a 50 gallon tank with a mix of lps, sps, and soft corals. Recently I have had problems with algae and figured out it was because of my phosphates. I put in some phos guard and that sorted out most of my problem. All of the algae went away except for brown algae on my aragonite. My dad’s friend has a 1500 gallon tank and is quite a serious reef junkie. He says I just need more flow. All my water parameters are good and the algae still won’t go away. My tank is doing about 33 times turnover per hour. Any suggestions ? Thanks

  • cheryl Boggs says:

    I just wanted to reply to JasPR, I don’t know how else to say this, but you hit the nail smack on the head. What you said could not be more true, and I just wish that all, in the hobby would understand that. We think that we have all of the control, with all of the bells and whistles, the this and the that, but in the scheme of all things, we have very little control of our beloved box of water. So the best that we can do is take care of our friends in that box of water as best we can and just ride the waves as they come.

  • gino505 says:

    I have a 150 Gal. Tank and I am very new to this hobby. My tank went from a little cloudy to solid sludge. I can see about 2 inches into the tank and then it looks like bright green light. When you look close to the glass I can see very small pieces of green substance (it looks like algae) I am currently trying to do the 3 days no light. Also I have been watching the tank build and i am sure my plumbing setup is one of the major causes to this headache. I am currently running bio balls with filters. Please help.

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