[Edward]

Hi,
is there a procedure, that can determine CO2 concentration in water at zero degree KH? The pH => KH table doesn't do it here.

My tanks have no issues with algae. Plants look great and fish are happy. Lemon Tetra (Hyphessobrycon Pulchripinnis) is breeding and raising fry without my intervention. The only problem I have is to control the CO2 level.

Details:

Black Neon Tetra
Bleeding Heart Tetra
Lemon Tetra
Cardinal Tetra

Hemianthus Callitrichoides
Glossostigma Elatinoides
Limnophilla Aromatica
Ludwigia Inclinata
Hydrocotyle Verticillata
Rotala Wallichii
Ludwigia Arcurata
Didiplis Diandra
Rotala Rotundifolia
Ludwigia Repens

RO water only

pH 4.1 - 5.2
TDS 150 - 250 uS
KH 0
NO3 5 - 20 ppm
PO4 0.25 - 1.00 ppm
K 10 ppm estimated
Ca2 10 - 20 ppm
Mg2 2.5 - 5.0 ppm
Fe TE (when Ludwigia Repens needs it)

K2SO4
KNO3
KH2PO4
MgSO4
TE (Fe EDTA 5%, Fe DTPA 2%, Mn 2%, Zn 0.4%, Cu 0.1%, B 1.3%, Mo 0.06%)

Mix of ratio
3.0 g CaSO4
1.0 g CaCl
1.5 g Soda
1.0 g MgSO4

It would be much easier if I could determine the CO2 concentration and keep it at 20 - 30 ppm.
Thank you,
Edward

 

[plantbrain]

Well no, you will not be able to determine at a KH of 0. pH test probes will also not work on pure water. You have some salts but no HCO3.

HCO3/CO2 is an acid base buffered system. Without the base, there's no pH/KH relationship.

The system will crash, it might not have done this yet for you, but you'll get burned at some point. And have dead fish.

My question to you is this: why do use RO water with no KH added back?
Bring it up to 2 at least.

Do this for your fish and for your plants. It will help and cause no issues with any fish or plants you mention or anyone's mentioned, even wild Rio ***** Discus, Altums etc do fine at a KH of 3.

Plants will use KH if there is not enough CO2 also. You may be hitting the right amount of CO2 for now, but that may not last long. If the plants do not have enough CO2 at some point, then the pH wioll rise dramatically since there's no base to balance it. Our blood has the same acid base system and is why we exhale CO2 as we oxidize the sugars we eat for energy.

Think about the acid base system, if you add a lot of acid and there's no base to balance it, then you can have very large pH fluctations. Potentially lethal to fish also.

Add some baking soda and raise the KH to 2-3 range, then add enough CO2 to get about 6.3-6.5 range.

All the fish will and can breed in this range.
Food and feeding will help the most in general with most breeding issues.

This will take care opf both fish and plants.
In general, taking care of the plants takes good care of the fish, the rest is mainly feeding.

Regards,
Tom Barr

 

[Edward]

Tom,
let me Thank you for the great help you have done for the aquatic plant community. Most of us would be still fighting algae today.

What condition is considered as a harmful pH crash is difficult to determine. In some parts of South American rivers water level rises several meters after rain fall. This water is filled with humid acid and CO2 from plant decomposition. Conductivity remains as low as 10 uS, pH around 5 and KH only in traces.

This is the environment where fish and plants flourish. For example Cardinal Tetra larvae will not develop and die in KH over 0.2 ppm, GH other then 1-2, pH over 6.0 (ideal 5.5) and conductivity other then 10-30 uS. So why not give plants and fish such conditions when RO allows it.

I've been keeping plants and fish in pH of 4.0 to 5.0 for the last 3 years with no problems. Some Tetras breed and raise babies and that is a prove how healthy environment they have. Most Tetras wouldn't be able to develop in KH of 2.

I have tried to increase KH with soda some time ago, but it was never ending struggle. Unstable pH fluctuations made it difficult. If there is a stable KH source like tap water then water changes don't cause variations. In my tanks with zero KH the pH fluctuation is 0.4 pH at most. Not much different then tanks with KH of 3. What mechanism is making the pH so stable without the KH buffering?

I believe that we are unnecessarily obsessed with the pH issue. The need to keep it in the range of 6.9-7.1 is a myth.

Thank you,
Edward

 

[gpodio]

I agree with Tom's suggestion, I would raise the KH above 0.

But there is a post at the end of this page that suggests a different equation to measure CO2 levels that doesn't use KH as reference. Perhaps it may work for you, but says you will need to have very precise PH readings:

http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html

Hope that helps
Giancarlo Podio

 

[fishfry]

I have always used straight R/O for my South American fishes. My rams have bred over and over, and now in my plant tank my toninas just love the KH ~1
to each their own I guess...?

 

[tsunami]

But Tonina's don't need such soft water to grow well. Tonina fluviatilis grows just fine in KH 4, GH 5 water:

Plus, many plants like Ammania gracilis and Nesaea pedicellata seem to prefer the slightly harder water. Plants do need calcium and magnesium, after all.

Carlos

 

[Edward]

gpodio

Thanks for the shortcut. I followed the article http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html and have done few experiments to realize later that the equation is probably wrong.

log(CO2) = pH(a) - pH(t) - 0.3

example 1
pH(a) 8, pH(t) 7, takes 5 ppm CO2

example 2
pH(a) 7, pH(t) 6, takes 5 ppm CO2

It is clear that this is wrong. From CO2/KH/pH chart http://www.eheim.com/technik_co2_dauertest.htm we can see it takes 10x more CO2 at 6 pH then at 7 pH.

I am hoping to find more options to measure CO2 levels.

Thank you,
Edward

 

[Edward]

tsunami

we don't use soft water because plants need it, but because it is more convenient when RO is used.

Controlling stable level of KH with chemicals is not easy task. Too many variables, too much time and work. So why not let the water stabilize naturally on it's own. I tried and it seems to be working just fine.

Yes, plants need Calcium and Magnesium. When Calcium test kit reads bellow 20 ppm Ca2, I dose CaCl, CaSO4, Soda and MgSO4 premixed dry in the right ratio Ca:Mg:Na.

Thank you,
Edward

 

[fishfry]

doesn't Flourish contain all the essential minerals that R/O takes out of the water?

 

[gpodio]

No Flourish is more of a generic all-round fertilizer/trace mixture made to suit many different tanks. Equilibrium is Seachem's product for RO water, and you will need to still raise KH and this only effect GH and some other elements not associated with water hardness.

Giancarlo Podio

 

[plantbrain]

My advice has been stated, I know the plants will look much better and as a result, so will the environment.

That's the oldest truism in the book: healthy plants= healthy fish.
Not the otherway around.

Plants do not do well in nutrient poor conditions, you might find a few there, but they are not growing actively.

A solution since you seem intent on using nothing in your water, fertilize the substrate.

I will have to completelty disagree with you about tetra's develoment requiring such soft water. I somehow managed with a KH of 5, one friend grew more tetra's and cories than he knew what to do with at a KH of 2-3.
I've bred several species, cardinals, lemon, rummy noses, I did not get huge spawns but I am not concerned with that.

So that is baloney they need such conditions to bred. You might get more fry yields, but they do breed at higher KH's.

What you read is a myth.
The pH thing is certainly a myth.

The first tenents of breeding: food food food food. Wide variety, live, often.

That is most often the biggest issue with breeding and fish health, next is a good enviroment, space, good O2, hiding places etc. Then maybe water chemistry at the end after other things have been exhausted.
I have yet to see any need to drop below a KH of 2.

But.......this is a plant forum, not a breeding forum.

The pH/KH chart stops being linear when you get to low KH values, even if you have a very accurate and specialize pH probes, you will still have an extremely difficult time measuring CO2.

Some folks can grow plants in water like this but............I can grow plants in non CO2 tanks also. Adding some Kh and some stable CO2 levels will help the plants a great deal.

Tonia does very well at a KH of 3.

You don't have to go to the Rio ***** to find acid balckwater fish or environments.

The plant Fest I put on takes folks to see plenty of blackwater and my fish from there also bred in my tanks at a KH of 3 even though they come from water that is no different than the SA fish.

KH=0, GH=1, pH=> 4.7-5.2 etc.

The tannins and peats can buffer the water some in these systems. Organic acids in sufficient amounts can keep the pH's stable relatively. But............plants would prefer CO2 over this situation. Plants will get some CO2 from microbial actions and sediments.

They will grow, but they grow much better in harderwater CO2 rich systems like springs. This is true in Florida as it is in South America.
Claus and I have said the same things for close to 7 years or so now independently of eachother.

There are not many submersed aquatic plants in the Rio *****, so you are creating something that is not present in Discus or tetra environment.
My best friend is an Ichythologist from Colombia that studies Amazonian fishes. They brought back a bunch of new knife fish species last month.

Bottom line is that you will do fine with both groups by raising the KH to 1-2, I prefer 3, another approach that has worked for me and others:

Keep the KH at 2 and then drop it down for breeding and development, harvest the eggs/fry and transfer to a small grow out tank and feed with an automatic feeder.

If you are going to bred with that as the main focus, do that, if you are going to provide a good home and a nice looking plants tank, focus on the plants, the fish will live for many years.

My point is to bend a little for each group, understand there is a great deal of myth out there, especially about soft water fish.

KH of 3 is often recommended, but KH 1-2 can work well also, just also long as it does not go to zero.

How often do you change your water? I change water with tap which is quite higher pH than the tank water and have for years. Typically 50-80% weekly. My fish have never had an issue, temp is the only thing I make sure is similar.

Hope this helps.

Regards,
Tom Barr

 

[plantbrain]

Are you using peat etc?
Also, what types of KH test are you using?

Try the double/triple/4x the volume of water sample and count the drops of KH solution, this will give you smaller increments.
So if a 5ml smaple = 1 drop = 1 unit of KH, adding one drop to a 10ml smaple will = 0.5KH. This only works perhaps to about 3x or so. Resolution bottoms out. I use a LaMott kit which is good to about 4-5ppm.

You might try this for measurements.
Make some standard solutions for KH.
Make some with the make up water and see if there is any pH differences between tank water and the make up water after they have both sat out on the table in a glass for 24 hours.

Both samples will have the exact same CO2 levels at equilbrium.

So if peat treat water has a KH o f 2 and pH of 6.0 while the make up water has a pH of 7.0.
You will substract the 1.0 unit and treat the CO2 injection like it's off by one unit.

This is not perfect, but it's close enough for plants/fish.

So instead of the chart being accurate, you have scaled it and calibrated it for usage with tannic acids in this case.

So you often hear that the peat will throw off the pH/KH/CO2 chart, this is true but............you calibrate the pH difference and use that measurement to scale it to find the CO2 level so there is a way around it.

So for the peat treated water has a KH of 2, a pH of 5.4 it will have a CO2 level of 24ppm.

Sneaky eh?
hehe, that's just the type of guy I am.



Regards,
Tom Barr

 

[Edward]

Hi

using minerals is not the problem here, you can see I am dosing already quite a few, KNO3, KH2PO4, K2SO4, MgSO4, Fe, Mn, Zn, Cu, B, Mo, CaSO4, CaCl, Soda and MgSO4.

The issue here is how to measure CO2 levels in water with too little KH.

One solution is to increase the KH by dosing dry Soda. This works only temporarily because plants remove part of it and RO water changes the rest. This creates too much of pH fluctuation. The remaining Na accumulation doesn't do good to the plants neither.

Second solution is to place coral, egg shell or some calcite mineral inside the aquarium filter. This has to be calibrated, / amount / flow / RO water changes / to keep the KH levels stable. Almost impossible to manage.

Third solution is to place coral or some calcite mineral between the RO unit and the aquarium, an add on filter. This will produce RO water with stable KH level. Water changes won't effect the KH as much and all will be more constant.

The third solution seems to be the most logical, but I am not sure, because unlike the aquarium water, the RO doesn't have CO2 to dissolve the coral calcium. What is your thought about this and what should I put in the filter?

The ideal situation would be, if we could run RO water over something to get the KH, Ca and Mg in it.

I change water in higher light aquarium every few days by letting RO flow of 60 gallon per day directly to the aquarium. An electronic automatic overflow valve under the aquarium disposes 10% of water every time water reaches the top. Can be ran indefinitely without supervision.

Other aquariums with low light I do water changes only when nutrient levels get out of whack.

There is little peat moss under 4" of silica sand to help plant roots with CEC.

Test kits I use for NO3, PO4, GH, KH, Ca and Mg are made by Nutrafin Hagen. TDS and electronic pH meter are made by Cole Parmer Labcor and calibrated with pH 4.0 and 7.0. The NO3 and PO4 kits I tested with calibrated solutions.

Regarding the calculation of the CO2 level from the two pH values, I am not sure if I understand it correctly.
at KH of 2,
from pH 8 to pH 7 => 5.9 CO2
from pH 7 to ph 6 => 5.9 CO2
from pH 6 to pH 5 => 5.9 CO2

Are you saying that it takes the same amount of CO2 to lower the pH one degree at pH of 8 as at pH of 6?

Thank you,
Edward

 

[gpodio]

I would just use baking soda to raise the KH. Just bring your RO water up to the desired KH before adding it to the tank, after a few water changes the KH in the tank will rise to match it, this should help avoid any big PH changes. I would bring it to 2 at least, 3-4 even.

Hope that helps
Giancarlo Podio

 

[plantbrain]

One solution is to increase the KH by dosing dry Soda. This works only temporarily because plants remove part of it and RO water changes the rest. This creates too much of pH fluctuation. The remaining Na accumulation doesn't do good to the plants neither.

Got cha, if the KH disappears, then you do not have ENOUGH CO2.
No beans about it then. Therefore this means that the plants do not have enough CO2 so they have no choice but to munch the HCO3.

Plants will only use CO2 if there's enough available to them and not touch the HCO3 at all.

So you do not have enough CO2 if they are removing your KH. That much is clear to me.

If your CO2 is too low, then we have issues with plant growth, potential for algae.

"

Second solution is to place coral, egg shell or some calcite mineral inside the aquarium filter. This has to be calibrated, / amount / flow / RO water changes / to keep the KH levels stable. Almost impossible to manage.

Even if this KH level varies some, so what? It'll balance out the pH more than what you now have. The CO2 from the injection will still be the same, so will the plant's CO2 demand and so will the CO2 concentration in the water.
Only the KH/pH will move.

Third solution is to place coral or some calcite mineral between the RO unit and the aquarium, an add on filter. This will produce RO water with stable KH level. Water changes won't effect the KH as much and all will be more constant.

Powedered dolomite would add KH and GH. So that might be an idea also.

The third solution seems to be the most logical, but I am not sure, because unlike the aquarium water, the RO doesn't have CO2 to dissolve the coral calcium. What is your thought about this and what should I put in the filter?

The CaCO3 will dissolve either way, faster if you bubbled CO2 in there like they do with a kalk reactors. But fine ground CaCO3 or dolomite will dissolve and add some KH/GH

"The ideal situation would be, if we could run RO water over something to get the KH, Ca and Mg in it. "

Dolomite.

I change water in higher light aquarium every few days by letting RO flow of 60 gallon per day directly to the aquarium. An electronic automatic overflow valve under the aquarium disposes 10% of water every time water reaches the top. Can be ran indefinitely without supervision.

Yes, we have several folks doing this with their tap water and works great, no work. Adding some GH and KH is easy though:

CaCl2 for Ca, MgSO4 for Mg, Baking soda for KH, these all dissolve instantly and can be made up ahead of time in the good ratios you want to shoot for.
These also cost peanuts.

Other aquariums with low light I do water changes only when nutrient levels get out of whack. There is little peat moss under 4" of silica sand to help plant roots with CEC.

Well, not too much is going on with CEC at this point, some organic matter and some bacterial cycling.

Regarding the calculation of the CO2 level from the two pH values, I am not sure if I understand it correctly.
at KH of 2,
from pH 8 to pH 7 => 5.9 CO2
from pH 7 to ph 6 => 5.9 CO2
from pH 6 to pH 5 => 5.9 CO2

Are you saying that it takes the same amount of CO2 to lower the pH one degree at pH of 8 as at pH of 6?

Yes. The relationship is linear(until you get near zero KH).

So at a pH of 7.0/KH8 with CO2 added and no tannins, I have 24ppm of CO2 through enrichment.
If I then add tannins(say from peat) and the pH drops to 6.0, I still will have the same CO2. The rate has not changed nor has plant uptake
The scale has been moved down, but the CO2 level is still the same.
You do not have enough CO2 based on the removal of KH/HCO3 from your tank.
This limits the plant's growth.

But you need some KH to get up to the linear part of the pH/KH scale no matter what. Even the above scaling will not apply once you bottom out the KH. There will no little way to measure the CO2 or pH at that point.

It's fine if you take good care to have a KH of 1, generally most folks add some buffering in their KH to 3 degrees but there's no reason the scale will not work below 3KH either, just not down less than .5 to 1 or so. Then you might bottom out the CO2 and remove all the KH and be back where you started.

Try 1 if you are chicken or 2. Folks breed wild Apisto's well at 2.

Regards,
Tom Barr

 

[Edward]

Got cha, if the KH disappears, then you do not have ENOUGH CO2.
No beans about it then. So you do not have enough CO2 if they are removing your KH. That much is clear to me. If your CO2 is too low, then we have issues with plant growth, potential for algae.

I didn't say this is happening to me, I am saying this method is unreliable in lower concentrations and this is not practical at continues RO flow through the aquarium. I am not going to dose soda manually every hour to keep the same KH from leaching out.

Even if this KH level varies some, so what? It'll balance out the pH more than what you now have.

In my tanks with zero KH the pH fluctuation is 0.4 pH at most. Not much different then tanks with KH of 3.

Powedered dolomite would add KH and GH. So that might be an idea also.

The dolomite CaMg(CO3)2 sounds like the most practical solution for RO systems. Even the ratio of Ca : Mg is right. I have already found a supplier and will give it a try tomorrow. Thank you for the information.

CaCl2 for Ca, MgSO4 for Mg, Baking soda for KH, these all dissolve instantly and can be made up ahead of time in the good ratios you want to shoot for.

Yes, this is what I've been doing for years now. CaSO4, CaCl, soda and MgSO4. I will try to replace this with the dolomite instead as a less time consuming solution and better water stability.

Yes. The relationship is linear(until you get near zero KH).

This is important information, Thank you.

Here, few pictures of my plants living at pH of 4 - 5 and KH of zero degree. Can you tell me what is wrong with the plants?

 

[tsunami]

The way you describe your tank and the "hassle" of adding KH/GH back into your tap water, it sounds like you have a continuous flow of RO water going through the aquarium. Why not just add Ca:Mg in the proper 3:1 or 4:1 ratio at every water change to raise it to KH 2, GH 3 for example? Then, you can measure your CO2 levels. It's really that simple. I don't understand why you refuse to do this --and I don't understand why you believe that raising them causes instability. My values are always rock solid on a certain dKH and dGH throughout the week.

There is no way of finding the CO2 levels without, as Tom said, a KH. To think that there is otherwise shows a lack of understanding of the chemistry at work in the water.

It boils down to what you want to do. Do you want to have a tetra breeding tank or have a beautiful planted aquarium with tetras? You chose what you want to focus on. Why grow plants in harder water when RO water is available? Because A) I think CO2 is more important than extreme water softness, B) my plants need Ca/Mg, and C) very, very few plants live in such soft water conditions in their natural environment. Unless I wanted to grow just Toninas (along with Mayaca and Phyllanthus fluitans, some of the only blackwater plant species... Rio ***** is VERY sparse on aquatic vegatation), I would harden the water so I can grow all of my other stem plants along with the Toninas.

IMO, your plants look healthy but on the very lean side in every sense from lighting to nutrients. That aquarium doesn't look very well lit judging by the fact that the Limnophila aromatica and Rotala rotundifolia are growing straight upward (under well light conditions, they will grow at an angle). The intense coloration should not be there if you have NO3 levels exceeding 10 ppm and PO4 levels exceeding 0.5 ppm --that would be very rich.

Although it is said that acidity has a role to play in plant coloration as well. I don't know how deeply people have looked into the acidity of the water column and its relationship with color.

Just playing devil's advocate for a moment, I'm keeping chocolate gouramis, green neon tetras, tube-mouthed pencilfish (N. eques). cardinal tetras, and rummynose tetras right now in tap water with KH 6 and GH 9. They have been in there since January. All is well, still... happy plants = happy fish is a philosophy I tend to use.

Carlos

 

[Edward]

Carlos,

The way you describe your tank and the "hassle" of adding KH/GH back into your tap water, it sounds like you have a continuous flow of RO water going through the aquarium. Why not just add Ca:Mg in the proper 3:1 or 4:1 ratio at every water change to raise it to KH 2, GH 3 for example? Then, you can measure your CO2 levels. It's really that simple. I don't understand why you refuse to do this --and I don't understand why you believe that raising them causes instability. My values are always rock solid on a certain dKH and dGH throughout the week.

That's right, you don't understand. If water of zero Ca, zero Mg, zero KH flows slowly, only 2.5 gal per hour, 24 hour a day through a 130 gallon aquarium, what happens to the KH? Slowly, slowly, hour by hour, the KH going to the toilet, gone, no more, out of the system. It's an automatic overflow disposal system.
It's not like you dump half of the aquarium water and fill it up in a minute and dose soda in it, done for a week. See the difference with the RO flow?

I don't understand why you refuse to do this

I do not refuse to use KH, see my previous post. Tom suggested a solution for RO users and I am following his advice. Additional dolomite filtration after the RO unit will make water with the KH as needed. I am working on it right now. After that the incoming RO water will have the same measurable KH all the time. Just like your tap.

My values are always rock solid on a certain dKH and dGH throughout the week.

Because you don't use automatic overflow water change with RO system.

There is no way of finding the CO2 levels without, as Tom said, a KH. To think that there is otherwise shows a lack of understanding of the chemistry at work in the water.

There are other ways of measuring CO2 levels in laboratories, but not for us, hobbyists. It's ok, I am getting dolomite, KH is coming soon.

It boils down to what you want to do. Do you want to have a tetra breeding tank or have a beautiful planted aquarium with tetras?

I don't want to shock you, but yes, I do have both at the same aquarium at the same time.

Some people don't want to believe it, just like few years ago people didn't believe silica didn't cause algae and phosphate didn't cause algae year later.

A) I think CO2 is more important than extreme water softness,

There is still the same amount of CO2 in softer water as in harder water.

B) my plants need Ca/Mg

All plants do. That's what CaSO4, CaCl and MgSO4 dosing is for. Ca and Mg have nothing to do with KH. You can have more Ca and Mg in zero KH water then you have now from the tap.

C) very, very few plants live in such soft water conditions in their natural environment.

That's why in my first post I listed plants I grow and later I attached even pictures for you to see that the plants are doing well.

That aquarium doesn't look very well lit judging by the fact that the Limnophila aromatica and Rotala rotundifolia are growing straight upward (under well light conditions, they will grow at an angle).

They have 4Wg, 10hours a day.

The intense coloration should not be there if you have NO3 levels exceeding 10 ppm and PO4 levels exceeding 0.5 ppm

There is NO3 10 ppm, PO4 0.5 ppm. Dosing for NO3 10ppm, PO4 1ppm.

I am not saying that the RO approach is better nor worse then the tap water. However, it is different. It does require different care, perhaps little more difficult then the tap. I will try the dolomite and see what happens.

It is an exciting hobby.
I'll be happy to read from you again,
Thank you for your time,
Edward

 

[Edward]

Rotala Wallichii, pH 4.0, NO3 20 ppm, PO4 0.15 ppm, light 0.9 Wg and bubbling

 

[plantbrain]

If you have a KH of zero, then measuring the pH will be extremely difficult to measure with a normal pH probe.

This requires different probes than the standard pH probes that come with pH monitors.

This can cause large errors in the pH readings.
What I'm not sure what effects the GH will have on pH readings but with out "any" buffering, there is no way to measure the pH of this mixture with CO2 being added with a standard pH probe.

I'll let you look into measuring pH of pure waters and let you decide for yourself. Many of the readings taken from the super softwater regions are not accurate due to this effect. A number of researchers addressed this with a special probe which are available from supply companies.

But on the practical level, the tank appears fine, you seem happy with it, why worry about CO2? Why are you concerned? Curious?

You might have hit a sweet spot but then again, the tank has been going long enough to have pH problems if this KH issue is really a problem.

It does not appear so in your case. The question is why?
I find this case very interesting.

I can tell it's working better than many might think at first............
But taking a look at the GH, and other parameters, general plant health appear good for species shown, seems like things are coming along fine.

FWIW, I have variations of about .1pH units at most for pH variation diurnally. Not much. But 0.4pH units is not bad either if you have intense pearling etc.

What we do not know and your question is what the CO2 level is.

You can try the backwards approach I suggested and assume that the delivery rate of CO2 will be the same no what KH value you have.

You agree with that?
So the CO2 rate added to the tank will be the same at a KH of 2 and pH of 6.3 will give roughly 30ppm or of CO2.

Okay, then you keep the same delivery rate of CO2 going and lower only the KH to zero.

The amount of CO2 being added should be the same in both cases and the plant's draw on/uptake of CO2 will be the same as well.

This might not be perfect but it'd be easy to do and give you a method of measurement. Adding HCO3 and CO3 increase's the solubility of free CO2 though...........

But I think it's interesting to consider adding no KH but everything else and crank CO2 in there. There is a minmum pH you can achieve using a weak acid like CO2 in pure water, 4.3 pH. So this is in your pH range that is acceptable with you.

Some humic acid may buffer this up as their end points might be higher.

Generally bacterial cycling will be much faster/higher rates etc at pH ranges closer to 6-7.5. But this is used to many of these fishes advantage, less bacteria, less stresses from these organisms.
But reminerlization rates are greatly slowed by low pH's.
That's why you get peat accreation in wetlands. Nothing breaks down because the low pH's.

Many algae cannot thrive at these low pH's and low KH's.
Diatoms and BGA's and other's fill their places, but this might help in some cases.

What I still curious is the plants, but pH in and of it's self is not really critical, CO2 is. I'm not sure that applies at a KH of zero, but it does pose an interesting dilema.

I'm trying argue for and against it here. I'm trying to come up with a few reasons as far as plants go, that it might not work and why it would.

But messing with parameters and seeing what happens is interesting and sometime surprising.

Many have done the RO water idea but failed, but did they fail(and myself) due to unstable CO2? Which you might not have to worry about due to a good estimation and observations of the plants?

Or did I/they fail due to not adding GH?

That's not clear as GH are like nutrients all of which you are supplying, even if lean to very lean, you still are not stunting some plants, N, K+ and P, traces etc are good as is the GH, the CO2 is unknown, I personally do not trust your pH meter readings.
But plants look decent, you seem happy and so do the fish.

Your pH variation certainly would shoot up if there was not enough CO2 being added during the day. In soft waters in nature I've found productive plant infested pools to go from 6 to 10 pH in less thna 12 hours. These pools have no measurable KH and need that special pure water probe.

So that says something telling about your CO2 levels.

Try the CO2 trick with a KH of 1 or two and reduce it down again, this will do no harm to these fish if you do it slowly. Then you can get a constant delivery rate and assume it to be the same. Also the tank water at equilibrium with air can also tell what the tannins influence are as will the amount of pH drop the CO2 alone adds.

Interesting topic. I suppose you might want to try some more with it, I sort of do, but have little need to make RO except for drinking water.

The other alternative for your routine is to do a 2x a week timer water change. Say 50 gal from the 130gal tank over several hours. Then dose back. A number of folks do this with excellent results with Altums and a horde of Discus. But they adjust the KH up. But as long as the CO2 is there for the plants, I'm wondering that even at a very very low KH, it may not matter that much.

As much as some of us may want to suggest you use some KH, I'd be interested in knowing more about this case at 0 KH.

I have very red R wallichii as well at very high NO3 and PO4 levels, some for a few other plants. Some plants do seem to redden up when you allow it to drop but I'm mighty happy with mine. NO3 testing is also an issue.

Go back throuhg and try testing some of these ideas I gave you, try to get a nice pure water pH probe for the meter. Double check things. Triple check things. Then try adding some KH to get your CO2(shoot for 25-30ppm).

Another issue, you do not have a lot of light but you add CO2, this is very important since light will drive Carbon uptake. At 0.9w/gal, there is much less CO2 demand and slower growth rates than at 5 w/gal.

But you might find a great improvement in growth by adding some K, PO4, more traces and a little KH(1 degree is fine since you are willing to work with it closely)

That is w/gal and not "w/liter" correct?
I've had a 60 gal with 54 w of PC lighting look quite nice, but I stay up around 1.5 to 2w/gal. The slower growth rates allow for better coloration in some cases you could argue also.

Alright enough of this yammering.

Regards,
Tom Barr