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?

21 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. 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!!

  3. 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!
      Thanks
      Tim

  4. 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..

    Thanks
    Sam

  5. 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.

  6. 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.

  7. 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.

  8. 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.
        thanks

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