GH,KH, water report and Chatgpt

[Mathieu]

Hi,

This my first post on this forum, after reading it for a year or so. I have hard water, and have always been struggling with all forms of algues since starting the hobby.
I am re-doing a tank. I have invested in an RO unit. I am still trying to decide how to proceed next:
1) easy solution to cut my tap water with 50/50 RO. Or more depending on my water supply.
2) or spend more money on remineralizer to have full control.
Green aqua had a recent video on water parameter, advising a GH of 5-6dH, a KH of 2-6dKH, while maintaining GH-2>=KH.
I do understand it is subjective, and personal. But they do have very nice aquascapes.
I have GH, and KH test kit, but I am colorblind so it is a struggle. I found the water report form my water suppliers. I have been trying to decipher it to get to GH, and KH, with the assumption that KH<GH for most tap water in the UK.
Like many, I am well confuse between GH, KH, alkalinity, total hardness. Especially, alkalinity and total hardness which, in my water report, is in both case a measure of concentration of CaCO3. How can you have a different name if it is a measure of the same compound?
I enclose the water report. Looking at mean values, I have an alkalinity of 283mg/l (second entry in the table), and a total hardness (GH?) of 225.81mg/l (quoted at the bottom).


So I turned to Chatgpt, and pasted it the water report. I am amazed at what it came up. I will share the transcript.
It came up with GH=11.6dH, and a KH=15.3dKH. and after cutting it with ~50/50 RO it would be 6dH, and 7.9dKH.
After reading so many post of the subject on the topic, I think I would agree with "him/her/it" 🙂.
Base on what I read online, I was reading my water report with the expectation KH<GH, as GH=KH+NKH so I am well confused. Does it seems correct then?

I am going to go with that, and see what happen. I won't meet the GH-2>KH. What is the drawback to have KH>GH?

I will use CO2 injection. The tank is for a classic planted with some tetra/rasbora and ammano/cherry shrimp.

Also I thought I could share my experience (some other may want to try too) and share the whole transcript below. Hopefully I am not breaking any rules.
The interesting bit, Chatgpt used Ca, and Mg concentration to workout GH.

=================================================================================================
Can you tell me the GH, and KH of my water supply based on the water report below?

ChatGPT said:​

ChatGPT
From the water quality report you shared:

General Hardness (GH)​

GH measures the total concentration of calcium and magnesium ions in water.

• Calcium (Ca): 83.08 mg/l
• Magnesium (Mg): 3.3 mg/l

Using the standard formula:
GH (°dH)=(mg/l of Ca7.14)+(mg/l of Mg4.34)\text{GH (°dH)} = \left(\frac{\text{mg/l of Ca}}{7.14}\right) + \left(\frac{\text{mg/l of Mg}}{4.34}\right)GH (°dH)=(7.14mg/l of Ca)+(4.34mg/l of Mg)
GH (°dH)=83.087.14+3.34.34≈11.64 °dH\text{GH (°dH)} = \frac{83.08}{7.14} + \frac{3.3}{4.34} \approx 11.64 \, \text{°dH}GH (°dH)=7.1483.08+4.343.3≈11.64°dH
So, your General Hardness (GH) is approximately 11.6°dH, which is considered moderately hard.

---

Carbonate Hardness (KH)​

KH is related to alkalinity, often expressed as the concentration of carbonate and bicarbonate ions. The report lists:

• Alkalinity: 273.5 mg/l

Using the conversion:
KH (°dH)=Alkalinity (mg/l)17.848\text{KH (°dH)} = \frac{\text{Alkalinity (mg/l)}}{17.848}KH (°dH)=17.848Alkalinity (mg/l)
KH (°dH)=273.517.848≈15.33 °dH\text{KH (°dH)} = \frac{273.5}{17.848} \approx 15.33 \, \text{°dH}KH (°dH)=17.848273.5≈15.33°dH
Your Carbonate Hardness (KH) is approximately 15.3°dH.

---

Summary​

• GH: ~11.6°dH (Moderately Hard)
• KH: ~15.3°dH (High Alkalinity)

4o

You said:​

So what is the difference between dH, and dKH?

ChatGPT said:​

ChatGPT
The terms °dH (degree of hardness) and °dKH (degree of carbonate hardness) are both units used to measure aspects of water hardness, but they describe different properties of the water. Here's how they differ:

---

1. °dH (General Hardness)​

• What it Measures:
  °dH refers to General Hardness (GH), which measures the total concentration of calcium (Ca²⁺) and magnesium (Mg²⁺) ions in the water. These are the minerals responsible for forming limescale in appliances like kettles and washing machines.
• Unit Conversion:
  1 °dH = 10 mg/L (ppm) of calcium oxide (CaO). It can also be expressed as a combination of calcium and magnesium concentrations.
• Relevance:
  GH affects fish and plant health in aquariums and indicates the "hardness" of water when considering its effect on household equipment.

---

2. °dKH (Carbonate Hardness)​

• What it Measures:
  °dKH refers to Carbonate Hardness (KH), which measures the concentration of carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions. These compounds act as buffers to stabilize pH by neutralizing acids.
• Unit Conversion:
  1 °dKH = 17.848 mg/L (ppm) of calcium carbonate (CaCO₃).
• Relevance:
  KH plays a critical role in maintaining stable pH levels in water. Low KH can cause pH swings, which can harm aquatic life, while high KH makes water more resistant to pH changes.

---

Key Differences​

Aspect	°dH (GH)	°dKH (KH)
What it Measures	Total calcium and magnesium	Carbonates and bicarbonates
Effect on Water	Determines "hardness" of water	Buffers pH stability
Impact	Affects limescale and mineral levels	Affects pH stability and buffering capacity
Units	°dH = 10 mg/L CaO	°dKH = 17.848 mg/L CaCO₃

---

Summary​

• GH (°dH): Measures calcium and magnesium—key for "hard" water.
• KH (°dKH): Measures carbonates and bicarbonates—important for pH buffering.

Both are important but serve different purposes in understanding water chemistry.
4o

You said:​

An alkalinity of 273.5 mg/l is how much dKH?

ChatGPT said:​

ChatGPT
To convert alkalinity from mg/L (ppm) to °dKH, you can use the formula:
°dKH=Alkalinity (mg/L)17.848\text{°dKH} = \frac{\text{Alkalinity (mg/L)}}{17.848}°dKH=17.848Alkalinity (mg/L)
For an alkalinity of 273.5 mg/L:
°dKH=273.517.848≈15.33 °dKH\text{°dKH} = \frac{273.5}{17.848} \approx 15.33 \, \text{°dKH}°dKH=17.848273.5≈15.33°dKH

Final Answer:​

An alkalinity of 273.5 mg/L is approximately 15.3 °dKH.
4o

You said:​

What is total hardness?

ChatGPT said:​

ChatGPT
Total Hardness refers to the measure of the concentration of calcium (Ca²⁺) and magnesium (Mg²⁺) ions in water. It is an important parameter in water chemistry because these minerals contribute to the "hardness" of water, which affects its behavior in industrial, domestic, and biological systems.

---

Components of Total Hardness​

1. Calcium Hardness (Ca²⁺):
   • The portion of hardness caused by calcium ions in the water.
   • Measured in mg/L (milligrams per liter) or ppm (parts per million).
2. Magnesium Hardness (Mg²⁺):
   • The portion of hardness caused by magnesium ions in the water.
   • Measured in mg/L or ppm.

Formula for Total Hardness (in mg/L as CaCO₃):​

To calculate total hardness as mg/L of calcium carbonate (CaCO₃), use the formula:
Total Hardness (mg/L as CaCO₃)=2.5×[Ca (mg/L)]+4.1×[Mg (mg/L)]\text{Total Hardness (mg/L as CaCO₃)} = 2.5 \times [\text{Ca (mg/L)}] + 4.1 \times [\text{Mg (mg/L)}]Total Hardness (mg/L as CaCO₃)=2.5×[Ca (mg/L)]+4.1×[Mg (mg/L)]

---

Total Hardness in °dH​

Hardness is often expressed in degrees of German Hardness (°dH), where:
1 °dH=10 mg/L of CaO1 \, \text{°dH} = 10 \, \text{mg/L of CaO}1°dH=10mg/L of CaO
The formula to convert Total Hardness (in mg/L of CaCO₃) to °dH is:
Total Hardness (°dH)=Total Hardness (mg/L as CaCO₃)17.848\text{Total Hardness (°dH)} = \frac{\text{Total Hardness (mg/L as CaCO₃)}}{17.848}Total Hardness (°dH)=17.848Total Hardness (mg/L as CaCO₃)

---

Classification of Hardness​

Hardness (mg/L as CaCO₃)	Hardness (°dH)	Classification
0–60	0–3.4	Soft
61–120	3.4–6.7	Moderately hard
121–180	6.8–10.1	Hard
>180	>10.1	Very hard

---

Example from Your Report:​

Your water report lists:

• Calcium (Ca): 83.08 mg/L
• Magnesium (Mg): 3.3 mg/L

Calculate Total Hardness (mg/L as CaCO₃):
Total Hardness=(2.5×83.08)+(4.1×3.3)=207.69 mg/L as CaCO₃\text{Total Hardness} = (2.5 \times 83.08) + (4.1 \times 3.3) = 207.69 \, \text{mg/L as CaCO₃}Total Hardness=(2.5×83.08)+(4.1×3.3)=207.69mg/L as CaCO₃
Convert to °dH:
Total Hardness (°dH)=207.6917.848≈11.64 °dH\text{Total Hardness (°dH)} = \frac{207.69}{17.848} \approx 11.64 \, \text{°dH}Total Hardness (°dH)=17.848207.69≈11.64°dH
This matches the values from your water report, which classifies your water as moderately hard.
4o

You said:​

I thought KH had to be less than GH?

ChatGPT said:​

ChatGPT
It’s a common assumption that KH (carbonate hardness) is usually less than or equal to GH (general hardness), but there are situations where KH can exceed GH due to the distinct chemistry of the water supply. Let me explain how this happens.

---

Why KH is Sometimes Greater Than GH​

1. KH and GH Measure Different Things
   • KH (carbonate hardness): Represents the concentration of carbonates (CO₃²⁻) and bicarbonates (HCO₃⁻), which act as pH buffers. KH doesn’t directly measure calcium or magnesium.
   • GH (general hardness): Measures the total amount of calcium (Ca²⁺) and magnesium (Mg²⁺) ions, which are responsible for "hard water."
2. Non-Carbonate Sources of Alkalinity
   • KH is influenced by carbonates and bicarbonates, which may be present in higher concentrations due to water treatment processes or natural geology (e.g., limestone-rich areas or artificial buffering agents).
   • GH is specifically tied to calcium and magnesium levels, which might be lower in certain water supplies.
3. Water Treatment and Additives
   • In treated water, carbonate and bicarbonate compounds may be added to stabilize pH or prevent pipe corrosion. This artificially increases KH without affecting GH.
   • Softening processes (e.g., ion exchange) may remove calcium and magnesium, reducing GH, while leaving KH largely unchanged.
4. Natural Geological Variability
   • In some water sources, natural conditions may result in high carbonate content (raising KH) and relatively low calcium/magnesium concentrations (lowering GH).

---

Specific Case: Your Water Report​

• KH: 273.5 mg/L alkalinity = 15.3 °dKH
• GH: ~11.6 °dH

Here, KH is higher than GH because:

• Your water likely has a high concentration of carbonates and bicarbonates relative to calcium and magnesium.
• This could result from natural geology (e.g., limestone deposits) or water treatment practices.

---

Typical Scenarios​

1. KH < GH (Most Common):
   • Typical in untreated or naturally hard water with high calcium and magnesium levels.
2. KH ≈ GH:
   • Occurs when both carbonate and calcium/magnesium levels are balanced, often in natural water sources.
3. KH > GH (Your Case):
   • Common in treated water or water from regions with carbonate-rich geology and relatively lower calcium/magnesium concentrations.

---

Summary​

Although KH < GH is typical, it's not a rule. Your water has high KH relative to GH due to elevated bicarbonates/carbonates, likely from geological factors or water treatment. Both values are independent, so it’s entirely normal for KH to exceed GH in certain circumstances.
4o

You said:​

In what proportion to mix my tap water with RO water to achieve an overall GH of 6dH?

ChatGPT said:​

ChatGPT
To calculate the mixing ratio of your tap water with reverse osmosis (RO) water to achieve a target General Hardness (GH), we can use the dilution formula:
Cm=(Ct×Vt)VmC_m = \frac{(C_t \times V_t)}{V_m}Cm=Vm(Ct×Vt)
Where:

• CmC_mCm: Target GH (desired final hardness, 6 °dH in your case)
• CtC_tCt: Tap water GH (11.6 °dH from your report)
• CROC_{RO}CRO: RO water GH (0 °dH, assuming pure RO water)
• VtV_tVt: Volume of tap water
• VROV_{RO}VRO: Volume of RO water
• Vm=Vt+VROV_m = V_t + V_{RO}Vm=Vt+VRO: Total volume after mixing

Since CROC_{RO}CRO is 0, the formula simplifies to:
Cm=Ct×(VtVt+VRO)C_m = C_t \times \left(\frac{V_t}{V_t + V_{RO}}\right)Cm=Ct×(Vt+VROVt)
We can solve for the ratio of Vt:VROV_t : V_{RO}Vt:VRO (tap water to RO water):
VtVm=CmCt\frac{V_t}{V_m} = \frac{C_m}{C_t}VmVt=CtCm VtVt+VRO=611.6\frac{V_t}{V_t + V_{RO}} = \frac{6}{11.6}Vt+VROVt=11.66 VtVt+VRO≈0.517\frac{V_t}{V_t + V_{RO}} \approx 0.517Vt+VROVt≈0.517
This means 51.7% of the total volume should come from tap water. The remaining 48.3% will be RO water.

---

Mixing Ratio​

The ratio of tap water to RO water is:
Tap Water : RO Water=0.517🙁1−0.517)≈1:0.93\text{Tap Water : RO Water} = 0.517 : (1 - 0.517) \approx 1 : 0.93Tap Water : RO Water=0.517:(1−0.517)≈1:0.93
For simplicity, you can round this to approximately:
Tap Water : RO Water=1:1\text{Tap Water : RO Water} = 1 : 1Tap Water : RO Water=1:1

---

Final Steps:​

• Mix equal parts tap water and RO water to achieve a GH close to 6 °dH.
• For higher accuracy, adjust slightly if needed based on testing.

Let me know if you want to refine this further!
4o

You said:​

What would be the KH of such a mix?

ChatGPT said:​

ChatGPT
To calculate the KH of the mixed water (tap + RO), we use the same proportional mixing formula as for GH:
KHmix=(KHtap×VtVm)+(KHRO×VROVm)\text{KH}{\text{mix}} = (\text{KH}{\text{tap}} \times \frac{V_t}{V_m}) + (\text{KH}{\text{RO}} \times \frac{V{RO}}{V_m})KHmix=(KHtap×VmVt)+(KHRO×VmVRO)
Where:

• KHtap=15.3 °dKH\text{KH}_{\text{tap}} = 15.3 \, \text{°dKH}KHtap=15.3°dKH (from your water report)
• KHRO=0 °dKH\text{KH}_{\text{RO}} = 0 \, \text{°dKH}KHRO=0°dKH (RO water has no carbonate hardness)
• VtVm=0.517\frac{V_t}{V_m} = 0.517VmVt=0.517 (proportion of tap water in the mix)
• VROVm=0.483\frac{V_{RO}}{V_m} = 0.483VmVRO=0.483 (proportion of RO water in the mix)

---

Calculation:​

KHmix=(15.3×0.517)+(0×0.483)\text{KH}{\text{mix}} = (15.3 \times 0.517) + (0 \times 0.483)KHmix=(15.3×0.517)+(0×0.483) KHmix=7.91 °dKH\text{KH}{\text{mix}} = 7.91 \, \text{°dKH}KHmix=7.91°dKH

---

Final Answer:​

The KH of the mixed water would be approximately 7.9 °dKH.
This means the water will have moderate carbonate hardness and a good buffering capacity, which should help stabilize the pH.

 

[Maf 2500]

Hey, welcome to the forum. I am by no means a water chemistry expert, but it took me a good few years to come to my current level of understanding of hardness/alkalinity relationships and I would say that everything the bot told you is 100% correct.

Yes, it is true that most tap water in the UK has GH greater than or equal to KH. For example mine is 2:1 (likely Rutland Water water) and many on limestone aquifier ground water report a 1:1 ratio. However, as the bot said, some geology will return different values or it could be treated with bicarbonates etc.

I noticed in the water report that your magnesium to calcium ratio is very low, something like 1:21, whereas most "experts" recommend a ratio nearer 1:4 or even 1:3 as optimum for plant health. It would be easy and cheap to address this by adding Epsom salts (magnesium sulphate) which would also raise the GH but not affect the KH at all. So, if I were in your shoes, I would reduce the ratio of tap water compared to RO (say 70:30 RO:tap) and add some Epsom salts to the mix. This would raise the GH relative to the KH and likely bring the two near parity. Sorry, too late at night for me to work out the maths for you on how much Epsom salts to add, but maybe Chatgpt can do it for you as they seemed to do a good job on your other questions!

 

[Mathieu]

Thank you for the suggestion., it make sense. I do have epsom salts ( that's how I started with this form, for the excellent fertiliser calculator).
I try to do the calculation myself first, I did enjoy chemistry at school.

 

[dw1305]

Hi all,

So I turned to Chatgpt, and pasted it the water report. I am amazed at what it came up. I will share the transcript.
It came up with GH=11.6dH, and a KH=15.3dKH.

In some ways you only need the conductivity value. Because the min. and max. values are different it tells you that your water varies through the year. I'd guess "saltier" in the summer.

The 637 microS value is water fully saturated with calcium (Ca++) and bicarbonate (2HCO3-) and almost certainly from a limestone aquifer. Conductivity doesn't tell you which salts (ions) you have, but they are ~always dominated by calcium and carbonate ions and your water (even the minimum value) has plenty of both.

Your water report lists:

• Calcium (Ca): 83.08 mg/L
• Magnesium (Mg): 3.3 mg/L

I noticed in the water report that your magnesium to calcium ratio is very low,

Which is useful, as often it doesn't quote a magnesium (Mg) value, there isn't a permitted legal limit for magnesium, so the water company can't be in breach of it.

You (or AI) can work out the dGH & dKH from those values (but dKH only as "alkalinity") - <"Some handy facts about water">. The full details for the calculations are in this thread <"Water Reports - the Basics, as I am not getting the math!"> derived from the calculations at <"Water Hardness">.

I thought KH had to be less than GH?

They are often linked because dissolved limestone (CaCO3) supplies 1 : 1 dGH (Ca++) : dKH (2HCO3-), but a compound like "Epsom Salts" (MgSO4.7H2O) or calcium chloride (CaCl2.nH2O) would just add dGH (the Mg++ or Ca++ ions) and potassium bicarbonate (KHCO3) just dKH (the HCO3- ion).

the maths for you on how much Epsom salts to add

You can always use the <"Nutrient Dosing Calculator">. Epsom Salts is actually an easy one to work out, just because it is 10% magnesium, so one gram of MgSO4.7H2O supplies o.1 g of Mg and that is a 100 mg, so 1g of Epsom salts in 10 litres provides 10 mg / l (which is "10 ppm") magnesium.

cheers Darrel

 

[Andy Pierce]

ChatGPT: KH plays a critical role in maintaining stable pH levels in water. Low KH can cause pH swings, which can harm aquatic life, while high KH makes water more resistant to pH changes.

I gotta disagree with ChatGPT on this one. In an open to environment aquatic system (your aquarium) ANY degree of KH will stabilise pH. You definitely do not need "high KH". We could also discuss how much stabilisation an aquarium's pH actually needs... my guess is "not much" based on reports of success in essentially 0 dKH setups.

 

[dw1305]

Hi all,

I gotta disagree with ChatGPT on this one.

The problem is that it is all over the WWW, so any AI is going to harvest it as <"a valid statement">.

ANY degree of KH will stabilise pH. You definitely do not need "high KH". We could also discuss how much stabilisation an aquarium's pH actually needs... my guess is "not much" based on reports of success in essentially 0 dKH setups

This is back to @Roland 's <"Soft water tank">

My 70l tank,gh5/6 ,kh1,K15ppm,No3 5/10ppm,po4 0.20ppm,tropica soil substrate,Sky light 60 light,co2 40ppm,fertilization -tropica green+ Aqua rebell micro special flowgrow

cheers Darrel

 

[Marcel G]

In an open to environment aquatic system (your aquarium) ANY degree of KH will stabilise pH. You definitely do not need "high KH". We could also discuss how much stabilisation an aquarium's pH actually needs... my guess is "not much" based on reports of success in essentially 0 dKH setups.

If I am not mistaken, truly zero KH cannot be achieved in practice, because CO2 will enter the water from the air and some of it will always be converted to bicarbonate (even if it is only a negligibly small concentration). Of course, if we add some strong acid to the aquarium, we'll have zero alkalinity at that point, but once the CO2 concentration in the air and water equalize, some minimal alkalinity will be created.

 

[dw1305]

Hi all,

If I am not mistaken, truly zero KH cannot be achieved in practice,

<"You need"> @Andy Pierce or @hax47 , but I think you are right.

I think the problems with both pH and dKH (obviously linked) is that you can't really extrapolate from hard, alkaline water to <"soft, non-buffered water">, where pH is never stable. A lot of people, particularly who keep marine fish or Rift Lake Cichlids, know that pH stability is important to them, but then apply that "knowledge" to other situations, <"where it isn't relevant">.

cheers Darrel

 

[Andy Pierce]

If I am not mistaken, truly zero KH cannot be achieved in practice, because CO2 will enter the water from the air and some of it will always be converted to bicarbonate (even if it is only a negligibly small concentration). Of course, if we add some strong acid to the aquarium, we'll have zero alkalinity at that point, but once the CO2 concentration in the air and water equalize, some minimal alkalinity will be created.

In general terms KH is 'acid titratable alkalinity' which is mostly from carbonate species under usual circumstances (can also have a small contribution from borate in seawater) and can not only be 0, but can also take negative values if you keep on going adding strong acid. CO2 entering the water from the air does not change KH because the amount of added bicarbonate from the hydration of the CO2 is exactly balanced by the amount of added H+ ions. CO2 (carbon dioxide) + 2H2O (2 water molecules) <-> HCO3+ (bicarbonate) + H3O+ (the free H+ winds up in this form). You can think of the added bicarbonate as being pre-titrated by the acidity. As you keep on decreasing the pH, the bicarbonate will all wind up as carbonic acid (H2CO3). In any event, it is the 'acid titratable' piece that matters because that's what sets the pH of the water and also provides the pH buffering capacity.

 

[hax47]

If we want to be very precise, KH is the sum of bicarbonate and carbonate, and alkalinity (what we actually measure in an aquarium) is the sum of a few species concentrations:
Alk = [HCO3-] + 2[CO3--] + [OH-] - [H+]
So, as @Andy Pierce said, CO2 will not raise the alkalinity, since the H+ is formed in equal amounts with bicarbonate when dissolved in water.

What is the drawback to have KH>GH?

They are often linked because dissolved limestone (CaCO3) supplies 1 : 1 dGH (Ca++) : dKH (2HCO3-), but a compound like "Epsom Salts" (MgSO4.7H2O) or calcium chloride (CaCl2.nH2O) would just add dGH (the Mg++ or Ca++ ions) and potassium bicarbonate (KHCO3) just dKH (the HCO3- ion).

Higher KH means that there are some other positively charged ions in water besides Ca++ and Mg++, which is often sodium. Sodium in high concentrations is not good for plants, so that is the drawback. KH>GH can be an indirect sign of high sodium levels.

 

[Mathieu]

Hi all, thanks for all the comments.

derived from the calculations at <"Water Hardness">.

This link is very very good. That's the one I was missing. Now, a lot of things make sense.

 

[Mathieu]

If we want to be very precise, KH is the sum of bicarbonate and carbonate, and alkalinity (what we actually measure in an aquarium) is the sum of a few species concentrations:
Alk = [HCO3-] + 2[CO3--] + [OH-] - [H+]
So, as @Andy Pierce said, CO2 will not raise the alkalinity, since the H+ is formed in equal amounts with bicarbonate when dissolved in water.



Higher KH means that there are some other positively charged ions in water besides Ca++ and Mg++, which is often sodium. Sodium in high concentrations is not good for plants, so that is the drawback. KH>GH can be an indirect sign of high sodium levels.

My water report shows a Sodium level of 43mg/l so I guess that would be considered high?

 

[Mathieu]

So if want to target: dGH=6, and dKH=4, by cutting my tap water with RO
By starting with tap water report (mean values):

• Ca = 82.1 mg/l, and Mg = 3.73 mg/l gives 82.1/7.14+3.73/4.34 = 12.4 dGH
• Alkalinity = 273.5/17.86 = 15.3dKH

To lower dKH from 15.3 to 4, is 4/15.3=0.26 dilution. To keep it simple let's round it to 0.25. So I need a ratio 1/4 tap: 3/4 RO.
I will end up with dKH=3.8, and dGH=3.1.
Now I need to raise dGH to KH+2, so 5.8, an increase of +2.7dGH
After dilution Ca = 20.5 mg/l, and Mg=1 mg/l
If I increase GH solely using Epsom salt: that's 2.7dGH*4.34=+11.7mg/l. Therefore I would end up with Ca=20.5mg/l, and Mg=12.7mg/l. A ratio of 1.6.
Is it to low? If so how low to go? It looks like I would also need to use some calcium chloride.
Also increase Mg +11.7mg/l, that's 117mg/l of Epsom salt. So for a 250 liters tank, 29 grams

 

[Maf 2500]

Personally I would halve the amount of Epsom salt you suggested. The GH would still be above the KH (but not by 2 points) and you would have a good ca:mg ratio.

I don't think the GH being 2 points above the KH is a metric worth worrying about.

 

[Marcel G]

CO2 entering the water from the air does not change KH because the amount of added bicarbonate from the hydration of the CO2 is exactly balanced by the amount of added H+ ions.

I apologize for my previous misstatement that "truly zero KH cannot be achieved in practice". For some incomprehensible reason I associated HCO3 with KH. What I meant was that when some CO2 goes into the water from the air, it increases the total dissolved inorganic carbon in the water (DIC), and since inorganic carbon takes on three forms in water (CO2-HCO3-CO3) based on pH, some of that CO2 is converted to HCO3. Therefore, as long as there is any inorganic carbon in the water (which will be constantly moving in from the air), there will always be some (even if only a small) portion of HCO3. Assuming, of course, that we keep the pH in the range normal for our aquariums (pH 5-8). However, I didn't realize that the H+ ions balance the negative charge of the HCO3-, which makes the alkalinity (KH) unchanged, and for that I apologize again and would rather not comment any further so as not to cause further confusion.

 

[hax47]

My water report shows a Sodium level of 43mg/l so I guess that would be considered high?

I think this level is somewhat high, but plants also have different sensitivity to sodium. I have 10-12 mg/l in my tap water, and most plants do fine in it. However, I also think that the toxicity of sodium depends on the presence of other ions like K+ and maybe Ca++ and Mg++. Adding more of these could decrease the toxicity, although I am not 100% sure of that. But there is some literature data (with terrestrial plants), and also some anecdotal ones.

 

[dw1305]

Hi all,

My water report shows a Sodium level of 43mg/l so I guess that would be considered high?

That is a lot.

Higher KH means that there are some other positively charged ions in water besides Ca++ and Mg++, which is often sodium. Sodium in high concentrations is not good for plants, so that is the drawback. KH>GH can be an indirect sign of high sodium levels.

Perfect. Only the carbonates of group 1 metals <"are soluble">, and it will sodium (Na), not potassium (K), I should have thought of that.

I think this level is somewhat high,

I think that is right, water derived from limestone aquifers tends to have <"some sodium (Na) content"> & <"https://assets.publishing.service.g...1/Wealden_District_Regional_Geology_V1.0a.pdf">.

but plants also have different sensitivity to sodium

They do, anything that <"grows in Lake Tanganyika"> should be fine.

Personally I would halve the amount of Epsom salt you suggested. The GH would still be above the KH (but not by 2 points) and you would have a good ca:mg ratio.

Same for me, probably 10% tap water (to supply calcium (Ca) and bicarbonate (dKH)) and a <"sprinkle of magnesium"> (Mg), job done.

cheers Darrel