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KH, GH & pH


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I have a very basic working understanding of pH, KH and GH. However there's something I don't understand so either I've misunderstood something or there's a gap in my knowledge. 

So as I understand:

pH is the measure of how acidic or alkaline the water is. 

GH is the measure of how much dissolved minerals there are.

KH is the measure of how much calcium there is specifically. 

 

As KH goes up so does pH and vice versa right? And as biological processes happen (bacteria and plant growth etc) calcium gets used up and therefore the pH and KH goes down right? 

So here is my confusion, it's often recommended to add sources of calcium such as crushed coral, aragonite sand, wonder shells etc to either the substrate or the filter or wherever. My understanding is this is to act as a buffer to stop the pH/KH getting too low and to make sure the plants and creatures that need calcium have it right? But what then stops this just making the pH too high? As far as I can tell people just add it in without worrying about dosing amounts etc, they just use for example crushed coral as the substrate right? Not weighing it out or anything.

I have really hard tap water (like 8.4ish) but it seems to lower fairly rapidly in my tank (lots of plants, wood, algae, aquarium soil) and seems to settle to around 7.2 which I like as it's fairly neutral and most fish seem to do alright in this. I try and do small water changes to keep it stable but say I go away for a few weeks and I want to stop the pH crashing I would need to add a buffer right? But then wouldn't this just make my water too high a pH, at least until the plants etc bring it down again? Or is it like the calcium only dissolves into the water when the pH starts to drop thus keeping it from getting too high? 

Feel like there's a piece of the puzzle I'm missing here. 

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I don't know if this will help or not, but it's my understanding that KH helps keep your pH stable.  Not enough KH = unstable pH.  For example, I've seen people who seem very knowledgeable about all this stuff say that if your water is high in KH then you can add all the leaves and tannins you want and it will have very little effect on pH, but with low KH tannins will lower pH.

If I got that all wrong hopefully someone will correct me, or at least explain it better.

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You're on the right track. You just have to understand the relationship between the starting pH and the speed with which the coral sand/aragonite dissolves. The rate of dissolution/solvency varies with pH. Drop a chunk of coral into strong nitric acid and it'll melt before your very eyes. Put the same piece of coral into water with a pH of 7.0 and it'll still slowly dissolve, just much, much more slowly. Imperceptibly even unless you look at it over weeks/months/years. On Instagram nickuhas has a short video of putting a piece of coral into an acid lack in Indonesia and showing the reaction. NICK UHAS on Instagram: “We put a piece of coral from Miami, Florida into an ACID LAKE located in East Java Indonesia. The result is amazing! Thank you…”

It's the relationship between pH and the amount of buffer you use that controls the pH and KH. If you have very acidic water the buffer will get eaten up very quickly but as it gets eaten the pH rises, slowing the dissolution/solvency of the buffer and thus slowing the rise of pH. Let's say you start out with a tank water of 5.5 pH. You put in a handful of aragonite and it melts away within days. The pH would have risen as a result. You put in a secnd handful of aragonite and maybe this batch lasts two weeks. Why? The water's not as acidic as it was. After two weeks that's gone and you add another handful and it now lasts a month. Why? The new starting point is at a higher pH.

I find in my tanks that the happy medium of buffering and pH happens at around 7.8 pH. The buffer (aragonite in my case) keeps the pH steady about there and the tank stays nice and stable there. The aragonite gets slowly consumed but I just add more from time to time. Everything stays nice and stable.

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On 9/9/2021 at 3:15 PM, gardenman said:

You're on the right track. You just have to understand the relationship between the starting pH and the speed with which the coral sand/aragonite dissolves. The rate of dissolution/solvency varies with pH. Drop a chunk of coral into strong nitric acid and it'll melt before your very eyes. Put the same piece of coral into water with a pH of 7.0 and it'll still slowly dissolve, just much, much more slowly. Imperceptibly even unless you look at it over weeks/months/years. On Instagram nickuhas has a short video of putting a piece of coral into an acid lack in Indonesia and showing the reaction. NICK UHAS on Instagram: “We put a piece of coral from Miami, Florida into an ACID LAKE located in East Java Indonesia. The result is amazing! Thank you…”

It's the relationship between pH and the amount of buffer you use that controls the pH and KH. If you have very acidic water the buffer will get eaten up very quickly but as it gets eaten the pH rises, slowing the dissolution/solvency of the buffer and thus slowing the rise of pH. Let's say you start out with a tank water of 5.5 pH. You put in a handful of aragonite and it melts away within days. The pH would have risen as a result. You put in a secnd handful of aragonite and maybe this batch lasts two weeks. Why? The water's not as acidic as it was. After two weeks that's gone and you add another handful and it now lasts a month. Why? The new starting point is at a higher pH.

I find in my tanks that the happy medium of buffering and pH happens at around 7.8 pH. The buffer (aragonite in my case) keeps the pH steady about there and the tank stays nice and stable there. The aragonite gets slowly consumed but I just add more from time to time. Everything stays nice and stable.

Thanks! I think I understood that, I was close-ish haha.

So it's unlikely you could use too much buffer as it would just slow down the rate of dissolution thus coming to equilibrium more quickly?

If I used a buffer in my tank and kept it steady at 7.8 or whatever, what would happen when you do water changes with higher pH tap water? (most people I see talking about using buffers talk about low pH tap water not high like mine) The pH would be increased because of the new water, but would it slowly drop back down to 7.8 from the typical biological processes the same (albeit slightly slower I guess) as if you did not include a buffer? Or would it stay around the new pH as the buffer stops it decreasing?

Is it possible to use a buffer to keep things steady at close to neutral? Like if you choose the right type of buffer? I like the idea of natural means to keep the parameters steady but most of the fish I plan to keep like it neutral to slightly acidic so I'd like to keep my water nice and steady at 7.0 ideally. 

Edited by KentFishFanUK
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On 9/9/2021 at 10:32 AM, KentFishFanUK said:

Thanks! I think I understood that, I was close-ish haha.

So it's unlikely you could use too much buffer as it would just slow down the rate of dissolution thus coming to equilibrium more quickly?

If I used a buffer in my tank and kept it steady at 7.8 or whatever, what would happen when you do water changes with higher pH tap water? (most people I see talking about using buffers talk about low pH tap water not high like mine) The pH would be increased because of the new water, but would it slowly drop back down to 7.8 from the typical biological processes the same (albeit slightly slower I guess) as if you did not include a buffer? Or would it stay around the new pH as the buffer stops it decreasing?

Is it possible to use a buffer to keep things steady at close to neutral? Like if you choose the right type of buffer? I like the idea of natural means to keep the parameters steady but most of the fish I plan to keep like it neutral to slightly acidic so I'd like to keep my water nice and steady at 7.0 ideally. 

I've never had high pH water but I would guess the natural processes would bring it down to the stable pH range. 

To get to a naturally buffered 7.0 or lower water, you're looking more at adding organics, like driftwood, peat, tannins, etc.  You'll still want a highish KH tokeep things stable at the lower pH though.

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On 9/9/2021 at 3:49 PM, gardenman said:

I've never had high pH water but I would guess the natural processes would bring it down to the stable pH range. 

To get to a naturally buffered 7.0 or lower water, you're looking more at adding organics, like driftwood, peat, tannins, etc.  You'll still want a highish KH tokeep things stable at the lower pH though.

Is it possible to get a balance of enough buffer to keep KH up and pH steady with enough organics to keep the pH from getting too high? If so is there a rough guide on how to go about it?

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You have a few concepts wrong.

PH you have correct.

GH is a measure of hardness. How hard the water is. Water hardness is made up of Ca and Mg. It could be all Ca, all Mg and any ratio of both from one extreme to the other. But GH is a measure of Ca and Mg.

KH is carbonate hardness. It's a measure of carbonates and bicarbonates. Nothing more. You can have carbonates like calcium carbonate, calcium bicarbonate, potassium carbonate and potassium bicarbonate. This is the buffering capacity of the water. Basically, KH maintains pH. Higher KH, higher pH range, lower KH, lower pH range.

Acids "eat away" at KH, effectively lowering it over time. And in turn, lowers pH over time.

Crushed coral would raise both KH and GH. It's a source of Calcium and carbonates. 

It's best to ignore pH and focus on KH. As KH determines relative pH ranges. 

Osmotic fluctuation is more problematic, KH fluctuation is osmotic fluctuation, pH fluctuation is not osmotic fluctuation.

PH can change while KH remains the same. 

GH fluctuation is also osmotic fluctuation.

 

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I believe most municipal water companies post the composition of their water. (Maybe different in England) You can find out your ratio of calcium to magnesium and what type of carbonate you have. I discovered that I have no detectable magnesium and all my calcium is in the form of calcium carbonate. 

Edited by Patrick_G
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On 9/9/2021 at 8:48 PM, Mmiller2001 said:

You have a few concepts wrong.

PH you have correct.

GH is a measure of hardness. How hard the water is. Water hardness is made up of Ca and Mg. It could be all Ca, all Mg and any ratio of both from one extreme to the other. But GH is a measure of Ca and Mg.

KH is carbonate hardness. It's a measure of carbonates and bicarbonates. Nothing more. You can have carbonates like calcium carbonate, calcium bicarbonate, potassium carbonate and potassium bicarbonate. This is the buffering capacity of the water. Basically, KH maintains pH. Higher KH, higher pH range, lower KH, lower pH range.

Acids "eat away" at KH, effectively lowering it over time. And in turn, lowers pH over time.

Crushed coral would raise both KH and GH. It's a source of Calcium and carbonates. 

It's best to ignore pH and focus on KH. As KH determines relative pH ranges. 

Osmotic fluctuation is more problematic, KH fluctuation is osmotic fluctuation, pH fluctuation is not osmotic fluctuation.

PH can change while KH remains the same. 

GH fluctuation is also osmotic fluctuation.

 

Thanks for clearing that up!

When you say KH maintains pH and higher KH means higher pH range - if you had neutral pH water and wanted it to stay there - would you want to increase KH? Is there a KH and GH value to aim at for keeping neutral and medium hardness? 

I haven't a clue what osmotic fluctuations are so I'll take your word for that part. If you are focusing on keeping KH steady instead of pH, how would you go about that? 

Although with your answer I feel like I understand GH and KH better, I feel like my understanding of how to manage them is even less! 

Say I was trying to breed a fish that prefers soft-medium and acidic to neutral water, and I was trying to keep it steady at those parameters but my tap water is hard with high pH, how would you go about it? I feel like I know what to do to bring it down (add organic material which is essentially adding acid right?) but then I get confused with how to keep it there long term without it either crashing or, if I add a bunch of KH buffer, increasing too much. 

Thanks for your reply, even though I'm getting confused I enjoy learning! I'll get my head round it eventually. 

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On 9/9/2021 at 5:01 PM, KentFishFanUK said:

Thanks for clearing that up!

When you say KH maintains pH and higher KH means higher pH range - if you had neutral pH water and wanted it to stay there - would you want to increase KH? Is there a KH and GH value to aim at for keeping neutral and medium hardness? 

I haven't a clue what osmotic fluctuations are so I'll take your word for that part. If you are focusing on keeping KH steady instead of pH, how would you go about that? 

Although with your answer I feel like I understand GH and KH better, I feel like my understanding of how to manage them is even less! 

Say I was trying to breed a fish that prefers soft-medium and acidic to neutral water, and I was trying to keep it steady at those parameters but my tap water is hard with high pH, how would you go about it? I feel like I know what to do to bring it down (add organic material which is essentially adding acid right?) but then I get confused with how to keep it there long term without it either crashing or, if I add a bunch of KH buffer, increasing too much. 

Thanks for your reply, even though I'm getting confused I enjoy learning! I'll get my head round it eventually. 

Keeping a KH between 1dKH and 3dKH should keep you around neutral if your substrate isn't active. Some substrates are active and reduce pH. 

You would soften tap water by cutting it with distilled or RO water. Example, your tap is 10dGH and 10dKH, 1 gallon of tap mixed with 1 gallon of Distilled water would cause 2 gallons of 5dGH and 5dKH.

To maintain certain numbers, you make your source water the parameters you want the tank to be, and simply by doing water changes, you eventually convert the tank to source water parameters. But, the tank has to be inert, no rocks leeching minerals ect.. Maintaining KH is the same, via water changes to replenish what may have been lost.

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On 9/10/2021 at 3:07 AM, Mmiller2001 said:

Keeping a KH between 1dKH and 3dKH should keep you around neutral if your substrate isn't active. Some substrates are active and reduce pH. 

You would soften tap water by cutting it with distilled or RO water. Example, your tap is 10dGH and 10dKH, 1 gallon of tap mixed with 1 gallon of Distilled water would cause 2 gallons of 5dGH and 5dKH.

To maintain certain numbers, you make your source water the parameters you want the tank to be, and simply by doing water changes, you eventually convert the tank to source water parameters. But, the tank has to be inert, no rocks leeching minerals ect.. Maintaining KH is the same, via water changes to replenish what may have been lost.

Thank you so much! 

So with some testing and number crunching I should be able to work out a water change schedule to target the KH and therefore pH that I want and if I have different tanks with slightly different requirements I could work out a schedule for each tank. 

Is there a trusted resource for KH value ranges to pH ranges? Would be handy to have a list or scale to refer to in case I need to work out schedules for different tanks. The only ones I can find a about levels of CO2. 

My planned breeding tanks will have inert substrates (though I will have plants and driftwood) but my community tank has Tropica aquasoil powder, which I'm assuming is not inert as it lowers pH considerably when first setting up your tank, would that affect it much? I'm actually not too concerned about the community tank but more out of interest. 

Edit: Also do you happen to know, on the serious fish website (my go to generally) when it quotes parameters the fish like it tends to list temperature, pH range and 'hardness' - is this likely to be KH or GH?  It sometimes shows it as ppm and sometimes as °H. 

 

Edited by KentFishFanUK
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On 9/10/2021 at 2:12 AM, KentFishFanUK said:

Thank you so much! 

So with some testing and number crunching I should be able to work out a water change schedule to target the KH and therefore pH that I want and if I have different tanks with slightly different requirements I could work out a schedule for each tank. 

Is there a trusted resource for KH value ranges to pH ranges? Would be handy to have a list or scale to refer to in case I need to work out schedules for different tanks. The only ones I can find a about levels of CO2. 

My planned breeding tanks will have inert substrates (though I will have plants and driftwood) but my community tank has Tropica aquasoil powder, which I'm assuming is not inert as it lowers pH considerably when first setting up your tank, would that affect it much? I'm actually not too concerned about the community tank but more out of interest. 

Edit: Also do you happen to know, on the serious fish website (my go to generally) when it quotes parameters the fish like it tends to list temperature, pH range and 'hardness' - is this likely to be KH or GH?  It sometimes shows it as ppm and sometimes as °H. 

 

Unfortunately, there's no chart with specific KH to pH ranges as each tank is different and there's no perfect condition to create such a chart. But eventually, you will get a feel of things and you will know the pH of each tank just by knowing the KH. Tannins, bioload and rocks can all alter KH, thus, every tank is different.

If that website is consistent with most others, hardness should be GH. We measure GH and KH in degrees for simplification. Basically, for both GH and KH, we can divide ppm by 17.9 to convert to degrees. 

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On 9/10/2021 at 2:10 PM, Mmiller2001 said:

Unfortunately, there's no chart with specific KH to pH ranges as each tank is different and there's no perfect condition to create such a chart. But eventually, you will get a feel of things and you will know the pH of each tank just by knowing the KH. Tannins, bioload and rocks can all alter KH, thus, every tank is different.

If that website is consistent with most others, hardness should be GH. We measure GH and KH in degrees for simplification. Basically, for both GH and KH, we can divide ppm by 17.9 to convert to degrees. 

Thanks! 

I will assume it's GH then.

Shame about the specific chart but at least I should be able to work out where each tanks KH needs to be to keep everything steady at the parameters it needs. Will probably have to label each tank to remember haha! 

Thanks for your time answering every question! Hopefully it might even help others who come across it understand as well

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On 9/10/2021 at 7:26 AM, KentFishFanUK said:

Thanks! 

I will assume it's GH then.

Shame about the specific chart but at least I should be able to work out where each tanks KH needs to be to keep everything steady at the parameters it needs. Will probably have to label each tank to remember haha! 

Thanks for your time answering every question! Hopefully it might even help others who come across it understand as well

Welcome.

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