# Dumb Question of the week



## Richard Dowling (7 Feb 2014)

If you were to lower the light and also co2 to match, are you still likely to get a light green drop checker? I'm assuming the drop checker measures free CO2 which hasn't been absorbed so by reducing uptake you can reduce co2 but still have 30ppm in the water column?


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## ivydree (7 Feb 2014)

Makes sense to me....

Let's see what the sepcialists say...


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## Edvet (7 Feb 2014)

30 ppm is not a function of light. If you have trouble getting enough CO2 in the water lowering lightlevels may help, but only because it lowers the CO2 levels needed


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## ceg4048 (7 Feb 2014)

Richard Dowling said:


> If you were to lower the light and also co2 to match, are you still likely to get a light green drop checker? I'm assuming the drop checker measures free CO2 which hasn't been absorbed so by reducing uptake you can reduce co2 but still have 30ppm in the water column?


This makes no sense. The objective of injecting CO2 is so that it gets absorbed. It's never an objective to measure 30ppm  just for 30ppm's sake, which is a fantasy anyway.

If you lower the light then you also lower the demand for CO2 so then 30ppm becomes irrelevant.

Whatever lighting level you use, add enough CO2 so that plants do not suffer CO2 deficiency and don't add so much that the fish are endangered.

People really need to jettison the 30ppm mindset.

Cheers,


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## Richard Dowling (8 Feb 2014)

So basically with lower light you can have a darker drop checker?

Sent from my XT890 using Tapatalk


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## ceg4048 (8 Feb 2014)

Well, in essence yes, but again, we keep trying to convince people not to put too much faith in the DC. The color of the DC should only corroborate what you see in the tank. Do not use the color as your only source of data. All you have to do is to browse the forum and you'll see plenty of cases where the DC is dark, but the plants are fine, or where the DC is dark and the fish are gasping, or, where the DC is yellow and the tank has symptoms of CO2 deficiency, or any combination of these.

First and foremost is to observe the growth and health of the tank an the well being of the inmates.

Too many people think of the DC as a one-stop-shop. Nothing could be further than the truth. It's just like the idiot lights in your car. I've had cars with the "Oil" light is on and I'd have absolutely no idea what it actually meant until I'd investigate further.

There are two main factors at play with CO2:

Factor #1 is the concentration of the gas in the water column just outside the leaf membrane.
The concentration of any solute in water (or any solvent) is another way of indicating what pressure that solute exerts on it's surroundings.
If the pressure of the dissolved CO2 in the water is higher than the pressure of the CO2 inside the leaf tissue then CO2 will move from the water across the leaf membrane into the leaf tissue.

So with CO2 injection we are force feeding the tissues by using high pressure to force the CO2 into the tissue. If the CO2 dissolution method is poor, and if the distribution method is poor, then the CO2 pressure near the leaf outer membrane does not increase sufficiently, the rate at which the CO2 moves across the membrane is low and the tissues do not get saturated with CO2 quickly enough. In other locations, like at the entry to the DC there may be fewer restrictions and so the CO2 may pass more easily into the DC than it does into the plant beds. So the DC can show a nice lime green color but yet the plants still suffer CO2 deficiency.

Factor #2 is the DEMAND for CO2, which, of course, is primarily driven by the amount of light bombarding the leaf. When PAR is higher, then the demand for CO2 is higher. So the consumption of CO2 in the leaf tissue is at a higher rate. If the consumption rate is too high, compared to the speed at which the CO2 diffuses across the cell membrane then this is the same as having a clogged fuel filter in your car. The supply will not equal the demand ad deficiency will result.

The plant has ways of dealing with low CO2 supply. Instead of solely depending on passive means, such as pressure differential across the cell walls, the plant will develop a more robust and active CO2 attraction and transport mechanism. The Protein called "Rubisco" is produced in higher quantities, but this is VERY expensive. In low tech tanks, you'll find that the percentage of Rubisco in the leaf is higher than in plants grown in CO2 enriched tanks.

When you add CO2 you immediately signal to the plant that it can reduce production of Rubisco and rely more on passive means of CO2 uptake, but if your CO2 delivery is inept, and if this results in a fall in the CO2 pressure, then the plant is screwed because it has already reduced the active uptake mechanism. To restart extra Rubisco production to compensate for the loss of pressure requires a couple of weeks and during that time the plant will suffer CO2 deficiency.

That's why newbies always have CO2 trouble. They think that all they need to do is turn on CO2, like pressing a button, but when you add CO2, you fundamentally change the behavior and physiology of the plant. Inconsistent effort results in physiological chaos, so that's why it has to be carefully managed. The fact that high gas pressure in the water is toxic to fish makes it even more imperative.

So if we just blindly focus on some mythical 30ppm number or DC color, instead of thinking about what is physically happening at the cellular level of plant and fish, we'll be in trouble very quickly. We have both tools to play with, gas pressure and PAR. Use the tools wisely and allow the DC to only corroborate what your observations are.

It would be a better mindset to rephrase your statement to:
"With a lower light, which reduces the CO2 demand, I can reduce the CO2 partial pressure in the water column and still maintain excellent plant health."

Cheers,


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## harryH (10 Feb 2014)

That is such an enlightening response from Clive. Though the OP (Richard) heads his post 'Dumb question of the week' it's only by asking such questions that we learn.
The more I read Clive's reply's the more I realise how 'dumb' I am . Such a great help.


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