Does Co2 Affect Hardness Or Ph?

I have two tanks, one is 80 litres, the other is 50 litres. The larger tank has a Fluval 106 external canister filter, the smaller tank has an Eheim Pick-up 160. Both filters are rated for bigger tanks than they are fitted to and each are cleaned at the same time, about every 3 weeks or when I see any sort of reduction in water flow from either.
I have the same substrate in both, gravel I collected from the beach and sterilised by boiling. I use root tabs, the same ones in both tanks and they are both planted with Amazon Swords, Java Fern and Hygrophilia polysperma. Other than the substrate and the plants there is nothing else in the tank; there is no driftwood, ornaments or rocks. Both are fitted with UV lighting wired to the same timer so they are both getting an identical light/dark phase.
I do a 20% water change one both tanks about every 5 days or so, it never goes longer than 7 days. I use RO/DI water with Seachem Equilibrium to remineralise. This is added before the water is added to the tank so both tanks are getting identical water.
I have small snails in both tanks which I control mechanically, I am not over-run in either tank.

My larger tank is stocked with zebra danios, male guppies and some juvenile Panda Corys. The smaller tank has just a couple of fully grown Bronze Corys and female guppies. Neither tank is anywhere near maximum stocking level and other than the danios they both contain identical fish.
They are all fed the same diet - flake food plus catfish pellets as the staple, they get frozen bloodworms and freeze-dried tubifex worms probably every other day and live grindal worms about once a week.

To all intents and purposes apart from size the two tanks are nearly identical, however there is one difference, the bigger tank has a CO2 system installed, the smaller tank does not. The java fern grows at about the same rate in both tanks, the Amazon Swords in the tank with CO2 have grown massive whereas those without have not yet bizarrely enough the Hygrophila Polysperma grows much better without CO2 than with.

One might think that with 2 near identical tanks my water reading would be the same but they are not.
This is the reading for the larger tank - GH - 120, KH - 80, Ph - 6.5, NO2 - 0, NO3 - 40
This is reading for the smaller tank - GH - 180, KH - 80, Ph - 6.7, NO2 - 0, NO3 - 40
There readings were taken immediately prior to a water change.
Both tanks are currently running at just over 29C, I have no heaters on at the moment, I'm in Greece and that is what the temperature is in my house.

The only really significant differences are that the general hardness is lower in the tank with CO2 although the Carbonate hardness is the same for both. The Ph is also slightly lower in the tank with CO2.
Where this gets really bizarre is that if I put a TDS meter on the smaller non-CO2 tank I get a reading of 147; this is about the same as I get on my third tank even though the stocking is different. When I test the tank with CO2 I get a reading of 546 and have always had similar readings.

Does anybody have any suggestions as to why my larger tank gives such a high TDS reading - is it the CO2 because I can't think of any other difference between the two tanks.

Sorry this is so long, I was trying to preempt the string of questions normally asked.

 

CO2 will lower PH. I don't use it myself, but CO2 with Water creates carbonic acid which lowers PH. At least that is my understanding. I don't know how any of that affects hardness though.

 

You said you took the readings prior to a water change. I'd have more info if you measure the pH after you let a sample of water sit for 24 hours in a cup, to outgas any CO2. Otherwise, you'll have to mention when was the pH measured: before the lights, during (how long after....) or after the photoperiod.

A TDS meter reads conductivity. All anions and cations contributes. When Carbonic acid dissociates, bicarbonate and carbonate ions are form. Conductivity is higher.

 

Don't know what it is, but you need to pantet it!
There is clear correlation between pH/KH/CO2, but GH - don't know how this is happening.

The best way to identify if CO2 is the cause is to switch the CO2 installation for a fortnight. You should then have the same test results as in the smaller tank if the GH drop is being caused by CO2 injection.

 

Does anybody have any suggestions as to why my larger tank gives such a high TDS reading

GH and KH are incorporated as part of TDS, but TDS also includes substances such as organic compounds (such as phosphate), the concentration of which is dependent on a tank's bioload, fish density, plant biomass, etc.

The injection CO2 does not directly affect GH, KH or TDS in any significant, detectable amount.

As to why your GH is lower: it could simply be the result of CO2 inducing greater growth in your plants, and therefore increasing the uptake of calcium and magnesium. It could be a higher concentration of phosphates and other compounds causing Mg, Ca and other divalent metal cations to precipitate out of the water column.

 

Don't know what it is, but you need to pantet it!
There is clear correlation between pH/KH/CO2, but GH - don't know how this is happening.

The best way to identify if CO2 is the cause is to switch the CO2 installation for a fortnight. You should then have the same test results as in the smaller tank if the GH drop is being caused by CO2 injection.

Yes, that was my plan. I have unplugged the CO2 and will leave it off for a while to see if it makes any difference. Everything in both tanks is disgustingly healthy. I'm sure that it is the CO2 which is making the difference although I am surprised at just how big a difference in readings I'm getting. TDS readings of 546 with CO2 compared with a reading of 147 without is a massive difference.

 

You said you took the readings prior to a water change. I'd have more info if you measure the pH after you let a sample of water sit for 24 hours in a cup, to outgas any CO2. Otherwise, you'll have to mention when was the pH measured: before the lights, during (how long after....) or after the photoperiod.

A TDS meter reads conductivity. All anions and cations contributes. When Carbonic acid dissociates, bicarbonate and carbonate ions are form. Conductivity is higher.

The water change has not been done for 5 days, I took these readings at lunchtime today, that's slap bang in the middle of the photoperiod. The lights (and CO2) come on at 8.30am and turn off at 5.30pm. The readings were done at about 1.30.

 

According to CO2/pH/KH chart (which I don't trust much....) you have over 40 ppm CO2, 5 hours into the photoperiod. Do you have a drop checker?

 

According to CO2/pH/KH chart (which I don't trust much....) you have over 40 ppm CO2, 5 hours into the photoperiod. Do you have a drop checker?

Yes, I do have a drop checker. I'm injecting only a bubble every 2 seconds or so.

 

Co2 at 30ppm usually drops the ph by 1. If the co2 was not running for 12hrs there should not be a difference in the tanks due to co2.
Anything that is different is there in the tank regardless of the co2 (see @Minnowettes post)

Also, not sure if your numbers are correct....
Using ro and equilibrium you would not have a kH at 80... so you would have to be adding something for the kH as well. This too can affect Tds.

Only way to truly determine is to do close to 100%wc, fill tanks with exact buffers to make them match exactly. Let’s the co2 tank run without it for a week and see if getting same results. Likely you will find it’s not the co2 causing the difference

 

Co2 at 30ppm usually drops the ph by 1. If the co2 was not running for 12hrs there should not be a difference in the tanks due to co2.
Anything that is different is there in the tank regardless of the co2 (see @Minnowettes post)

Also, not sure if your numbers are correct....
Using ro and equilibrium you would not have a kH at 80... so you would have to be adding something for the kH as well. This too can affect Tds.

Only way to truly determine is to do close to 100%wc, fill tanks with exact buffers to make them match exactly. Let’s the co2 tank run without it for a week and see if getting same results. Likely you will find it’s not the co2 causing the difference

This gets more confusing by the day. As suggested by RSababady I left everything else alone and switched off the CO2 three days ago, the TDS reading has now risen from 546 to 621. The reading in the non-CO2 tank has remained pretty much the same, now 158, up just 11 from 147.

I have absolutely no explanation for this, all I can say with a fair degree of certainty is that it is not the CO2 causing such high readings. I am tempted to strip the entire tank down and start again. It could do with rescaping anyway; the panda corries currently have nowhere to hide apart from under plants so maybe some rock work would be a good addition.

One last thought, when I originally bought the tank it had a built in internal filter containing charcoal, clay balls of some description and ceramic rings. I removed this filter about 18 months ago and replaced it with the Fluval 106 external canisters. I discarded the carbon but kept the ceramic rings and the clay balls which were added to the Fluval. I'm just wondering whether the clay balls may have some bearing on this; they are now 3 1/2 years old. Is it possible they are in some way affecting the TDS? I can think of nothing else that is different between the two tanks.

 

TDS is the measurement of all inorganic and organic dissolved solids in your water.
So the question is what is leaching minerals into the water? The water in both tanks comes from the same source and therefore cannot be the source of the problem.
Possible sources are something you have in your filters - like old carbon (I know some people will say this is a myth, but in your case, I would compare apples to apples)

The other thing that comes to my mind is evaporation - how much water is evaporating from each tank? Do they both have covers? Keep in mind the more the evaporation, the higher the concentration of...everything

One more thing - just check if there are any metals that may be leaching - especially lead.

Ahhh.... stick your elbow into the tanks (one by one) while standing on the floor. If you feel a twinkling / twinging effect that means that you have an electrical leak in the tank - i.e. there is a current flowing through the water which will is known as electrolysis - this will raise the no. of ions in the water - and raise your TDS.

"....and that is all I have to say on this subject" Forest

 

TDS is the measurement of all inorganic and organic dissolved solids in your water.
So the question is what is leaching minerals into the water? The water in both tanks comes from the same source and therefore cannot be the source of the problem.
Possible sources are something you have in your filters - like old carbon (I know some people will say this is a myth, but in your case, I would compare apples to apples)

The other thing that comes to my mind is evaporation - how much water is evaporating from each tank? Do they both have covers? Keep in mind the more the evaporation, the higher the concentration of...everything

One more thing - just check if there are any metals that may be leaching - especially lead.

Ahhh.... stick your elbow into the tanks (one by one) while standing on the floor. If you feel a twinkling / twinging effect that means that you have an electrical leak in the tank - i.e. there is a current flowing through the water which will is known as electrolysis - this will raise the no. of ions in the water - and raise your TDS.

"....and that is all I have to say on this subject" Forest

RSababady - I'm pretty sure that evaporation is not an issue, one of the tanks has a lid but the other does not. It is the tank with the lid which is giving the high reading; water does evaporate from the open topped tank faster than from the covered one, if evaporation were the issue it would be the open topped tank that would be affected more than the one with the lid.

I cannot see that leaching metal can be a problem; I have never put anything in there which is metal, not even lead strips to anchor the plants down, they have all been rooted direct into the substrate.

I will try the electrolysis test but in the meantime I think I will be removing the clay balls from the filter and see if that gives any improvement.

 

TDS is the measurement of all inorganic and organic dissolved solids in your water.
So the question is what is leaching minerals into the water? The water in both tanks comes from the same source and therefore cannot be the source of the problem.
Possible sources are something you have in your filters - like old carbon (I know some people will say this is a myth, but in your case, I would compare apples to apples)

The other thing that comes to my mind is evaporation - how much water is evaporating from each tank? Do they both have covers? Keep in mind the more the evaporation, the higher the concentration of...everything

One more thing - just check if there are any metals that may be leaching - especially lead.

Ahhh.... stick your elbow into the tanks (one by one) while standing on the floor. If you feel a twinkling / twinging effect that means that you have an electrical leak in the tank - i.e. there is a current flowing through the water which will is known as electrolysis - this will raise the no. of ions in the water - and raise your TDS.

"....and that is all I have to say on this subject" Forest

Organic compounds have no charges (do not dissociate), therefore will not increase the hardness the way TDS meter works. A solution of glucose in water will not increase conductivity even though glucose is a "dissolved solid".

I could only speculate if I say the source of high TDS in injected tank is coming from the substrate. If everything else is the same, the pH in CO2 tank should be lower. That alone can affect solubility of certain rocks. One can argue that if that's the case, KH should be higher as well. I am not sure about that until I know for sure how KH test works exactly

 

Organic compounds have no charges

Right - I was actually thinking about the inorganic minerals....

 

Organic compounds have no charges (do not dissociate), therefore will not increase the hardness the way TDS meter works. A solution of glucose in water will not increase conductivity even though glucose is a "dissolved solid".

My mistake! You're definitely right. Not being a chemist, I think I alternate between the more specific term "compound" and the more general term "matter" far more freely than the definitions allow.

I think Mr. Wong at advancedplantedtank.com words it more accurately what I was originally attempting to write:

"Organic waste also raises TDS; unlike what most aquarists think, organic waste doesn't transform magically into just ammonia. Breakdown of organic waste goes through intermediate stages that releases carbohydrates, lipids, proteins, nucleic acids into the water column."

 

Summary for ease of understanding

Tds is a measure of “stuff” in the water, not any specific “stuff”, but general “stuff”
There is something that is creating more “stuff” in that tank...

Chances are with the co2 off plants are using less ferts, thereby increasing the Tds higher then it would normally climb

Ph “drop” with co2 in is not the same as a ph “swing”. It’s a reading that temporarily changes (reads lower) but the actual ph is still the same...

 

Summary for ease of understanding

Tds is a measure of “stuff” in the water, not any specific “stuff”, but general “stuff”
There is something that is creating more “stuff” in that tank...

Chances are with the co2 off plants are using less ferts, thereby increasing the Tds higher then it would normally climb

Ph “drop” with co2 in is not the same as a ph “swing”. It’s a reading that temporarily changes (reads lower) but the actual ph is still the same...

My understanding is that OP is reading high TDS in the tank with CO2. I don't quite understand what you said "with CO2 off plants are using less ferts, thereby increasing the TDS"
I am even more confused with the last statement: "pH drop with CO2 is not the same as a pH swing". I believe CO2 is exactly the reason why planted tanks (injected or not) experience pH "swing" on a daily basis. And not just our tanks but ponds and lakes as well.

 

The pH does "change" but not actually. The KH that causes the pH stays the same, and the pH only changes because of different levels of CO2 in the water.

 

I believe CO2 is exactly the reason why planted tanks (injected or not) experience pH "swing" on a daily basis.

That's correct. CO2 does produce a real and tangible change in pH, but it acts on pH in a way that does not exert harmful effects on livestock.

I think there's an element of confusion in how some resources interpret the relationship between pH, KH and CO2. I think most of this occurs because, for the longest time, pH was used as a proxy for KH due to a lack of titration equipment (necessary for measuring KH) accessible to aquarists in the early days of the hobby.

The logic was that a change in KH (i.e. carbonate salt concentration) will produce a corresponding change in pH, and for the most part this is true. Significant fluctuations in KH will disrupt the osmoregulatory process in fish, and so pH could be used as a proxy for gauging KH when KH could not in itself be measured directly.

CO2 somewhat complicates this relationship. When CO2 is in a solution of water, a very small number of CO2 molecules will hydrolyse to form carbonic acid:

CO2 + H2O <=> H2CO3

Carbonic acid can dissociate into bicarbonate and a free hydrogen ion (the latter of which decreases pH):

H2CO3 <=> HCO3- + H+

Bicarbonate itself can further dissociate into carbonate with an additional free hydrogen ion (the latter of which further decreases pH):

HCO3- <=> CO3(2-) + H+

As pH is a measure of the concentration of hydrogen ions, it can be seen that carbon dioxide can exert a real and tangible change in pH.

Carbonates do react with these free hydrogen ions (with a corresponding decrease in KH) and buffer the solution against a decrease in pH. However, because only a very small number of CO2 molecules (Tom Barr quoted it as 1 in 400) hydrolyse to form H2CO3, the impact of CO2 on KH is negligible, and undetectable to our hobbyist KH test kits.

So ultimately CO2 does exert an effect on pH, and while it does exert an effect on KH, it does not affect carbonate salt concentration to a degree that disrupts osmoregulation.

 

aniroc minnowette & culprit gave a bit more to the long story, I was trying to keep it short and sweet for ease of understanding...

Also possible I misunderstood, but believe OP mentioned that the co2 tank had a higher Tds to start.
Then mentioned that the Tds is continuing to climb with co2 off, hence my throwing out the idea