When should my CO2 come on?

It’s a good idea to start your CO2 roughly an hour or so before your lights come on. The reason for this is simple.

When your lights come on, you want your plants to hit for floor running. What I mean by that is you want your plants to begin growing instantly. As the CO2 build up over the 60 minutes before the lights are on your levels begin to rise (remembering the carbon dioxide has been off for maybe 16 hours) and your plants take complete advantage of this. Good levels of CO2 and lighting (don’t forget your fertilisers) = happy planted aquarium.

By contrast, having your CO2 come on when your lights start up, you delay the growing process as it takes roughly 1 hour for CO2 levels to hit that perfect 30ppm level.

In addition, turn your CO2 off 1 hour before your lights go off. There will be sufficient CO2 in your tank for that time for your plants to continue growing.

When do you turn your CO2 on and off?

35 thoughts on “When should my CO2 come on?”

  1. Hi Richard, I have my co2 come on about 2 hours before lights on, then I have it turn off an hour before lights out. I like to make sure I definately have enough co2 before lights on.

  2. My drop checker is green when lights turn on. It goes a lime green once lights have been on about an hour.

  3. Hi. I have a 190ltr tank quite heavily planted and a CO2 diffuser on all the time!! Not really considered turning it off, having said that it is only providing about 6 or 7 bubbles a minute. The tank is really healthy and plants and fish are all flourishing. Is it ok to leave on – you have me worried now!!

  4. 6-7 bubbles per minute is pretty much fruitless and at that rate you may as well turn it off. If you used a drop checker with bromo blue, you would fine the checker would stay blue all the time 🙁

    Are you using a drop checker?

    On a 190L tank, aim for about 2 bubbles per second and then see how the plants go.

    The reason why your tank is OK at night is due to the fact the CO2 is having no effect. It’s great that your plants are healthy, and there’s no reason for them not to be, but it sounds like you’re running what I would term a low tech planted tank.

    Does that make sense?

    1. Hi Richard. Thanks for this.
      Yeh, it does make sense. I have kept the CO2 low as I was a little worried about adding too much and it becoming detrimental the fish? Can you advise on this please?
      I will up it when the lights are on and turn it off at night and see how things go.
      Not heard of a drop checker before (by the way, I am a complete novice!). I will look them up!

    1. You can pick up a check valve at Amazon for ebay relatively cheap. Or try your local pet store. It takes all the guess work out.

      1. it’s not the number of bubbles that count as all bubble counters are different

        You need a drop checker to monitor the levels and you increase your CO2 flow rate according to the change of colour in the drop checker

        1. Totally agree! I got trashed on a FB group for having 2 bubbles /sec in a 50l tank DESPITE having zero algae, great plants, spawning fish AND a green dropchecker!
          I left the group!

  5. I admit – I am a total newbie to CO2 injection..though I had been running a
    25 gallon tropical tank with a lot of plants, and guppies, mollies etc since the last good 8/9 years.

    I am thinking of taking the plunge into the CO2 injection scenario, and would like help from you experienced team of aquarists.

    I have few questions regarding the same :-

    1) Do I keep running CO2 24/7 ? or switch off at night ??
    2) Do I run aeration pump at night (while the CO2 is switched off?)

    My photo period is from 6am to 10am then from 6pm to 10pm – Do I switch off the CO2 during the times in between ?? its not pitch dark in the room though!

    Your help will be greatly appreciated..


  6. CO2 only needs to be on when the lights are on. Ask yourself this question – why would you need CO2 when the plants are expiring (releasing CO2, not using it).

    You can run an airstone at night if your fish are gasping. They will only gasp if your CO2 levels, plant mass and water distribution are wrong.

    Having a gap in your photo period is a total waste of time and has no effect on algae. It also WILL NOT help your plants. A certain company thought a gap in the photo period was a good idea in the 1990s. Let’s not forget 50 years ago, smoking was good for you and helped expand your lungs…

    So have your lighting on for a max of 8 hours per day in one stint.

  7. I have huge problems with green spot algae in my tank. After reading a few of these articles on here iv reduced the lighting from 9 hours to 8 hours and iv moved the drop checker down to the bottom left corner rather then top right.

    The main problem i seem to have with my Co2 drop checker is it always seems to be blue to dark green on a morning (cant really tell what it is) and then by the time the afternoon comes, its yellow.

    Lights come on at 10:00am and go out at 6:00pm now but i did have this before and granted green spot algae was less but i still had it appearing after a week.

    Co2 comes on 2 hours before the lights and goes out 2 hours before the lights turn off as well running roughly around 1.5 bps using an atomizer.

    im afraid if i move the drop checker to the bottom left corner of the tank it will most certainly be blue and increasing the Co2 amount will end up being too high for the fish?

    1. It’s a case of trial and error Craig, but if your dropper is blue in the morning then I suggest your water distribution isn’t up to scratch. Improve that and it should go green quicker.

      If and only if that doesn’t have the desired effect then you need more CO2. But remember 95% of all algae related issues are down to not enough CO2 – simple as that.

  8. Hi, I’ve just set co2 system up on my tank. I don’t put the lights on til around 5 til 11pm because the tank is in a well lit room next to loads of windows. But I’ve been running the co2 from 10am til 10pm. Do I need to have have the lights on earlier.

  9. Hi
    I have a 300l tank quite heavily planted with jbl co2 system. have set up timers as you suggest today, and drop checker shows green, co2 bubble counter is now on around 2 bubbles per second, have green hair algae on plants? what am I doing wrong?

      1. many thanks for your reply, I am using aquagro grobeam 600 ultima led lighting x 2 and an arcadia led strip, in your opinion is this sufficient?
        drop checker is now showing light green. lights on for 7 hours at the moment.adding ferropol 24 daily. 1 bubble per sec.

  10. I recently purchase a co2 injection system with a 5lb tank. It’s all running good so far.
    It’s running on my 65 gal planted aquarium. I have it set at 3bbs.
    However my drop check never got out of the blue. Should I increase the bubble count?
    Also, how long after the co2 is turned on before it reads green on my drop check.
    Should it take 4, 6, or 8 hours or more?
    Thanks for any help you can give me.

  11. running CO2, i have read you don’t need to run it four 8 plus hours as long as it is steady running i run mine for five hours a day seven days a week, drop checker is reading blue. i do use a bubbler thingy in hour after the CO2 goes off. and it runs the rest of the day and into the night and cuts off one hour before my CO2 comes on, is this right ?
    CO2 starts two hours before the lights come on and goes off five hours after that, 6:00 am to noon. plants are ok could be better i dose fertilizers to . thank you for any help you can give me.

  12. How long should I have my CO2 diffuser on? And this should be turned on when there is no light, for example when its dark. Should this be turned on for a couple of seconds, mins, hours? Does it make a difference if I have a bubble counter connected to it.

  13. Hi Richard, how are you? Came across this blog searching for how much CO2 should I dose to a 372 LTR tank and how often should I leave the lights on.. is 20 watts sufficient for the tank? On my plants the leaves grow about an inch distance from each other on a stem.. what could be wrong? I am a newbie 🙂 I also put 10ml KNO3 solution to the tank twice a week.. this is my first planted aquarium using DIY CO2 diffuser, sterilized garden soil, KNO3 and some 25+ varieties of aquatic plants on individual pots at the moment later I want to create an aquascape out of it.. at present there just one Betta fish in it to keep control over any unwanted creature in the tank.. your input will be appreciated.. thanks 🙂

  14. Hi I’ve installed co2 n I’ve kept a bubble counter 2 bubbles per second but I’m unable to see any reaction in The diffuser ?

  15. Hello Richard, when my day light goes off can I turn on a red or blue moon light on or this harm the plant or co2.

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