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Too little light or too little CO2?

Discussion in 'Lighting' started by scottward, Feb 25, 2015.

  1. scottward

    scottward Guru Class Expert

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    How to tell the difference?


    On my 100g planted tank, full EI dosing currently, only 2 rows of T8's are on (so only 120W total T8 lighting). CO2 is cranking through.


    No significant algae (except brown diatoms, I presume due to tank still maturing). Plants are growing very very slowly though. New healthy growth is evident at the tips of stems, but it is so so slow.


    So is this a CO2 limitation or do I need to add a bit more light? How do I tell which is the limiting factor?


    The T8's are about 18 months old now, but I'm sure Tom once said to run 'em till they drop....


    Thoughts?


    Scott.
     
  2. Christophe

    Christophe Subscriber
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    How are you measuring the effectiveness of your CO2 dosing? Regular pH drop of 1.0 or better is a good indicator, drop checker is slower to respond, but can also be ok.


    How tall is the tank? How far from light to substrate?
     
  3. scottward

    scottward Guru Class Expert

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    Not measuring it at all at the moment. I will take a pH measurement before the CO2 comes on, and then take another one close to when the CO2 turns off. I aerate during the non-CO2 period so presumably I can then use the 1.0 pH drop as a guide?


    The tank is 20" tall, allowing say 2 inches for the substrate height and the lights being slightly raised although directly sitting on the top of the tank inside a hood, I'd say light to substrate is about 18-19".
     
  4. Christophe

    Christophe Subscriber
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    If you have a regular drop of at least 1.0 by lights on, not much algae to speak of, good surface ripple (see photos of Tom's tanks), you might be good to increase your light. If you can add another bulb, and at the same time, raise the fixture a bit, you can increase it in a controlled manner & see how it goes.
     
  5. scottward

    scottward Guru Class Expert

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    Ok so if I measure the pH of the water immediately before the CO2 comes on (1pm) and then again immediately before the lights come on (2pm), and the pH has dropped by 1 unit between these 2 points, I am in good shape CO2 wise?


    I guess it would be wise to measure pH during the photoperiod (2pm - 10pm) too though, to ensure this 1 drop is being maintained....
     
  6. Tug

    Tug Lifetime Charter Member
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    Two T8's at 18-19" can provide anywhere from 20-35mmol PAR, so it is low lighting. With older bulbs say, 20-25 PAR.
     
  7. scottward

    scottward Guru Class Expert

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    Cheers Tug. So the incredibly slow growth, even from the stem plants, is probably because the lighting is incredibly low (I..e .light limited, not CO2 limited)?
     
  8. Tug

    Tug Lifetime Charter Member
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    You are able to do quite well under low light when CO2 levels are sufficient.


    Slow growth, as long as it's healthy, isn't a bad thing. Cheers.
     
  9. Christophe

    Christophe Subscriber
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    Yes, should be good.

    Couldn't hurt. Not absolutely necessary though. If you got down 1 point in an hour, you shouldn't have a problem maintaining that.


    Light-limited is a good place to be -- less work, less opportunity for things to go wrong.
     
  10. Dennis Singh

    Dennis Singh SynKing!

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    Question is what plants are you growing? I've been able to grow a lot of things with shop lights alone kind've like your setup. Now we've got par data making things a lot easier. A lot of time people overkill on lights, even myself.
     
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