Way to control CO2 that is more forgiving and uses light as limiting growth factor?

jeremy v

Guru Class Expert
Apr 17, 2008
There is a new way of injecting CO2 that I have been thinking a lot about trying that I have never heard anyone else use and I was wondering if it has been tried before.

These are the good points that I would expect from this setup. It would work using all the same controlling mechanisms that many are already using on their tanks, just in a different combination than I have seen before. It would even work on DIY CO2 tanks with the addition of an inexpensive pH controller for a "middle of the road" setup that was much less costly than a full CO2 system and also a lot more forgiving of the variability in production you get from DIY CO2 setups for less algae issues. It would automatically compensate for moderate fluctuations in CO2 (lack or excess) without triggering algae because it actually tries to always ensure that light is limiting growth. Let me know what you guys think.

The only bad point I can see so far is that one light bulb might not last as long from being switched on and off more often each day depending on how sensitively it is all set up initially.

An ideal explanatory setup would be something like a 75 gallon tank with 3 54w t5HO lights above the tank. Set it up for bi-level switching so that only two lights can be turned on or so all three can be turned on at once.

This is the what I am thinking. Just run the CO2 directly to the CO2 diffuser in the tank via a needle valve set at a good level for adequate CO2 for the present tank conditions, volume of plants, light level, level of surface agitation, etc. Have the first two t5HO lights turn on and off each day via a timer and nothing more. Then instead of using a pH controller to turn the CO2 on and off like many do, use it to turn on and off the third t5HO light. When the pH goes up beyond a preset level (indicating that the CO2 reserves in the water are low) the third light turns off which would drop the tanks CO2 requirements, and when the pH drops beyond a certain point (indicating that the CO2 injection might be outpacing the uptake by the plants) the third light turns back on in order to increase plant uptake and keep the CO2 within a tighter range all day.

I would say that this setup would ideally be dialed in so that under normal circumstances the third light in the above example would be on about half of the time overall during any one day. The daily lighting in this example would have a base level of 54w + 54w + 54w/2 (since only on half of each day) = 134 watt equivalent all day.

If the pH was always too low so the third light was always on, the tank would be getting all three bulbs all day for a total of 54w x 3 = 162 watts all day, which is about 20% more light than the base level.

If the pH is always too high the third light would never come on at all so the tank would get only two bulbs of light each day for a total of 54w x 2 = 108 watts, which is about 20% less light than the base level.

That means that the whole tank could vary in CO2 injection accuracy or requirement by approximately +/- 20% every single day, and the tank would be totally unaffected by those swings because the lighting could do all the compensating in order to keep light the limiting factor for growth and prevent algae from being triggered or fish being hurt by having too much CO2. The same setup with only two bulbs instead of three and having one turned on and off with the controller would even give more of a variability range, which might be perfect for a DIY CO2 type setup where CO2 fluctuates a lot more.

I could see this setup also working as a good visual indicator of CO2 levels, because you would be able to know that if the third light was off for most of the day that the needle valve might need to be adjusted to increase overall CO2 a little bit, and if the third light was on almost all day that would tell you that the plant mass might have been trimmed back too far (for instance) and now the CO2 needs to be dialed back a little bit in response. It would also tell you visually if the CO2 was too low in the morning and too high later in the day etc. if the third light never came on in the morning hours and then stayed on all evening before lights off. That would tell you to ideally turn the CO2 on a little earlier in the morning before lights on (to boost morning levels a little more) and to also dial it back just a little bit (to reduce evening CO2 levels a little bit) to even the CO2 levels out more during the lights on period.

Let me know what you guys think or if you have tried it or heard of it being tried before.

Have a good one, Jeremy


Plant Guru Team
Lifetime Member
Sep 23, 2007
South Florida
Hi Jeremy,

Nice idea......

My concerns would be:

1. PH is not a direct relation to c02 available in the water. So, the meter/controller would be controlling the lights NOT based on c02 concentration in the water but also on other carbons,etc that affect PH.

2. IME my meter/controller always seems pretty stable in terms of reaching the set point rathery quickly and staying there, so it would seem the third light would be on a lot. Plus, as per point #1 the 20% extra would be on and maybe not due to enought c02.

3. The PH probe is stuck in one location and thus the light will be controlled by the c02/ph in THAT location.

4. Those with MH cannot use this as the bulbs cannot be turned on/off quickly.

Hook it up to a c02 meter that measures dissolved c02 and you have got it......



Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
I'm probably just confused, but my understanding is that a light limited tank is one where all of the growth drivers are present in more than adequate amounts at all times, so only the light limits the growth. So, we add "too much" of all of the nutrients, following the EI method, so none of them ever limit the growth. And, CO2 is one of the nutrients, so we try to keep the level of CO2 in the water above the plant's requirements to meet the growth rate that the light drives them at. If the CO2 level goes higher, but not so high as to damage the fish, that's ok, just so the CO2 level doesn't go lower than the plants need for the light driven growth rate. If this is all correct, I can't see what the advantage would be to varying the light level throughout the day.


Guru Class Expert
Jun 8, 2007
One issue is that plants don't adapt instantly to varying co2 levels. If the ppm is around 30 and suddenly in one day drops to 10, the plants don't just slow growth by 2/3, they seem to totally stop growth since they're not able to adapt that fast. So I don't know if this would be of much benefit, other than maybe alerting the aquarist that it's time to do something about the co2. When I was doing diy co2 on my low light 30g, things seemed to chug along fine until bam, one of the bottles stopped producing. It seemed that anything above about 15ppm was fine, as soon as you get below that, it was time to do something about it or it would be trouble. But then again, it was a very low light tank, so it was already light limited all the time as long as the plants were growing. As soon as the co2 would drop below that, any light was too much because the plants just screeched to a halt. On a tank with higher light, maybe it would be of benefit, if only to slow algae growth if there were co2 issues.