# Co2 - light cycle relationship



## jah191

I have a 48W T5 over a 20g tank with pressurized co2 setup. I have a solenoid on my regulator (Milwaukee all in one) and would like to put it on a timer. Most things I have read suggest turning off the co2 at night - which is what I plan to do.

2 questions:

Should I run my light all day - around 12 hours or so?

If I should do less - like say 6-8 hours - should I turn off the co2 when the T5 light goes off or when I turn all the lights off in my house (ie when I go to sleep.) Same question with regards to turning on the co2 - should I turn it on when the sun comes out or when I turn my lamps on?

The aquarium is not near a window but is in a living room that gets a decent amount of light in the mornings. Thanks.


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## Seattle_Aquarist

Hi jah191,

First of all I see you are new to APC.....welcome!! This is a great site with excellent information and friendly people to help you if you need it. I am glad you joined us!

I don't use a solenoid on my CO2 system but a lot of people do. Most people seem to start the CO2 about 1/2 hour prior to the aquarium light coming on and turn off the CO2 when the aquarium light goes off.

With the 2 watts per gallon (WPG) you have on your tank 12 hours may be too much light, especially at first and / or if you aquarium is lightly planted. My tanks have about 2 WPG and I typically start with about 6 hours per day and gradually increase about 1 hour per week as the plant growth fills in. I watch carefully for algae growth, especially for the first 30 days during the nitrogen cycle. I and cut back the lighting period if strong algae growth starts to set in.


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## jeanlery2010

I understand that aquarium plants need sufficient light and Carbon dioxide in order to grow well. I also have 2 watts per gallon on the tank for more than 6 hours. However, the only thing I knew is to give my plants their basic needs in any way to sustain them their daily requirements.


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## tanker

My CO2 comes on 1/2 hour *AFTER* the lights come on and off 1/2 hour before lights go off.

PS--My lights are on 10 hours per day.


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## niko

The most practical approach is indeed to pair up the CO2 injection with the most light.

Turn the CO2 on about an hour before the lights come on. That way the CO2 will have time to build up a little. And when the light is on the plants will have both CO2 and light available.

Turn off the CO2 about an hour before the light goes off. That way the CO2 will start to decrease before the lights are off. When they are off then everything slows down almost to a stop. All nutrients are used very slowly, there isn't enough CO2 and light and the algae has very little chance to survive.

Best setup - have strong lights 2-5 hours in the middle of the day. Inject CO2 during that period only. Before and after that period use only low light enough to see the aquascape well.

Or you can run your lights 10-11 hours a day, inject CO2 24/7 (or only during the day) and play with the tank all the time - fertilizing, changing water etc. For many people that's more fun than just having a hands off aquascape. 

--Nikolay


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