What can affect CO2 dissolving?

[Tom]

What are the factors (apart from temperature) that could possibly affect how easily CO2 will dissolve in water or lower the availability to plants? Some say hardness can be a factor, some say outright it's not. I do find it interesting though that I had no CO2 issues at a GH of 1, whereas now I can't get rid of the BBA and Staghorn at GH 20 ish while injecting twice as much. I have kept it stable, and have very good circulation.

Is there anything in tap water or any circumstances at all that can affect it?

 

[Johno2090]

Water hardness is one.

 

[Tom]

Thanks Johno - It would make perfect sense for hardness to affect CO2 based on current problems, but then I have seen people adamant about it having nothing to do with it. What's the reasoning for it, do you know?

 

[gmartins]

But GH is all about Mg and Ca. I do not think it should affect CO2 dissolution.

may be wrong though

cheers, G

 

[Garuf]

Altitude, volume of water, surface area, size of bubbles.

 

[Tom]

Thanks 🙂 Has anyone got any other thoughts or explanations either way?

 

[Johno2090]

I think it's something to do with the saturation level of the water. As in hard water has more saturated in it but I'm sure the 'holy' CEG will be here soon to put us right.

 

[ceg4048]

Tom,
You cannot simply pick any factor to explain the difficulties you are having with a particular tank. The factor that you select must show complete consistency in it's effects. One also has to be able to control all the other factors in order to draw the conclusion that factor X has an effect. There could be 10 different reasons why the previous tank did not suffer as much algae as this one, starting with the types of plants that were/are in each of the tanks. PAR differences would be the next possibility. Tank depth, distribution method, regulator/hardware, plant biomass and scape details are other contributing factors.

If you were to do a poll and ask who has had BBA or staghorn and what water was being used, I'm certain that you would be hard pressed to find any correlation between the occurrence of these algal forms and the hardness of the water being used.

Pressure and temperature are by far the most important factors in the dissolution rate of any gas in any liquid. The variations that occur as a result of the carbonate content of the water cannot be seen at this level. The presence of other solutes in the water have no effect on solubility of CO2.

Cheers,

 

[George Farmer]

Pressure and temperature are by far the most important factors in the dissolution rate of any gas in any liquid

Does that mean with cooler water less CO2 injection is required to reach a given level compared with warmer water?

This is interesting, as I also assume a plant's nutrient requirement goes up as temperature increases due to the increase in metabolic processes?

 

[Tom]

Thanks Clive,

I'm simply trying to go through each possibility changing one thing at a time, seeing if there is any difference. Trial and error. It was just my thought that if the water was jam packed with loads of solids, it might be harder to dissolve something into than if it had a very low TDS. If that's not going to be the issue, then on to the next possibility. It's not as if the CO2 has been all over the place in terms of stability either (just increasing every so often), so in that case the only option is more. I doubt the flow/distribution could be improved. The spray bar squirts it out along the surface, and the bubbles go down the front glass and back up along the substrate. When I put the drop checker in, it's solid yellow 24/7 at the substrate at the back.

What would be your next move if if water source won't affect anything?

As comparing to previous tanks depends on so many things, this had an Elite Mini filter in the front-right, and only 1bps CO2. Most of which ended up at the surface. Much more heavily planted, same size/height, almost 1.5x more light (similar T5 setup), Riccia, Stems, Hydrocotyle and moss (and a very fast growing tank). Bit of a mess really!




Thanks again

 

[ceg4048]

Pressure and temperature are by far the most important factors in the dissolution rate of any gas in any liquid

Does that mean with cooler water less CO2 injection is required to reach a given level compared with warmer water?

Hi George,
Yes this is correct! Check out the chart shown below:
(Chart courtesy of engineeringtoolbox.com)

This chart shows the solubility of CO2 as a function of temperature. On the vertical axis this indicates how many grams of CO2 will dissolve in 1Kg of water at 1 atmosphere of pressure.

At about 24 deg.C the solubility is about 1.5 grams per Kg of water.
At about 30 deg.C the solubility is about 1.25 grams per Kg of water.

So just lowering the water temperature by 6 degrees increases the solubility of this gas by 20%.
However, the solubility of the gas also is a function of the pressure. That function is Henry's Law which states that under constant temperature:
Gas Pressure = (Solubility constant)*(Concentration) or P=k*C
This is a simple ratio and if you solve the equation for the constant k => P/C then this means that the ratio of a gases pressure to it's concentration is constant. So if you increase the pressure by 20% then the solubility immediately increases by 20%.

What does this all mean? It means in relation to your question that all you have to do in a warmer tank is to tweak the needle valve slightly and you can immediately compensate for the loss of solubility due to increases temperature by simply increasing the injection pressure. So if I had excellent CO2 at 24 degrees with my needle valve set to 1.5 psi and then I got discus and had to increase my tank temperature to 30 degrees, then I just need ti increase my needle valve setting by 20% i.e. 0.3 psi to make the new working pressure 1.8 psi. That keeps the solubility of the gas about the same.

This is interesting, as I also assume a plant's nutrient requirement goes up as temperature increases due to the increase in metabolic processes?

Yes this is true, but that then means you must increase the CO2 even more than just compensating for the solubility decrease due to increased consumption requirements.

I'm simply trying to go through each possibility changing one thing at a time, seeing if there is any difference. Trial and error. It was just my thought that if the water was jam packed with loads of solids, it might be harder to dissolve something into than if it had a very low TDS

Actually, the only solid dissolved in water that has a serious impact on CO2 solubility is salt, NaCl. That's why CO2 is less soluble in ocean water than in fresh water.

It's not as if the CO2 has been all over the place in terms of stability either (just increasing every so often), so in that case the only option is more

Well, if we're seeing BBA then the tank is disagreeing with you, right? Because BBA is only ever causes by poor/unstable CO2. Even if it were true that higher TDS caused lower gas solubility, then that would only mean that you should get a low CO2 algae, like hair, not a fluctuating CO2 type algae, because the solubility although lower, would still be constant.

The focus here I reckon is to determine what mechanism is causing the instability. A dropchecker won't always show you instability because it's response time is so slow.

Is it possible that your regulator is allowing surging? Do you have another that you could try? Is the dropchecker yellow at the time of lights on? Have you measure the pH profile throughout the photoperiod in order to plot pH v time? I think you need to do this to absolutely determine if there is an instability.
The species in your old tank all appear to be those having a lower CO2 compensation point. Are there the same species in your new tank?
BBA also is a tenacious species, so that even if you fix the conditions it won't just go away. Have you tried blackout and/or Excel overdose to eliminate it from the tank completely?

Just some ideas to kick around...

Cheers,

 

[scottward]

An interesting read.

I'm not using a pH meter to control the CO2 bubble rate - I just set it and that's it (like most of us I guess).

My tank is also not heated or chilled at the moment. The temperate is quite variable. One day it's 28, the next day it's 32 and so on.

I have a question.

Given that I am setting the CO2 at a fixed rate of injection in this manner, does this mean that my CO2 levels will fluctuate in correlation with fluctuating tank temperature due to:
a) the solubility factors detailed above
b) the change in metabolic rate of the plants impacting CO2 levels

?

If yes, could this be sufficient to trigger a red algae infestation?

In winter things aren't so bad, because I "set the bar" higher by using a heater (even the peak ambient temperatures are lower than where I have the heater set so naturally I acheive temperature stability).

In the absence of an expensive chiller to set the bar lower, would I perhaps be best using a heater in summer and setting it at, say, 30C to achieve better temp (and CO2?) stability?

Scott.

 

[ceg4048]

Hi,
Yes, these are all possibilities but are not likely to be the sole cause. They are strong contributory factors, but if the tank suffers BBA/Staghorn or other CO2 related algal types then you'd probably want to look elsewhere such as at the basic injection rate, injection timing, regulator performance and flow/distribution, lighting, and so forth. It's impossible to have an absolutely rock solid CO2 concentration value all photoperiod, and minor variations during the day are not really a problem. Also remember that the first few hours of the photoperiod are the most critical, so if you have a weakness at this time it will produce a greater chance of triggering a bloom than issues that arise much later in the photoperiod.

You can take a series of pH readings during the day as discussed in this thread viewtopic.php?f=37&t=19499&start=10 to see if you've got significant variations and then troubleshoot from there.

Cheers,