Masha
Aquarium Advice FINatic
Being a beginner with very little knowledge of chemistry, I've been reading up on Ph, Kh and Gh. I've tried to compile a list of facts and explanations suitable for people like me. It would be fantastic if some of the more knowledgeable members here could have a look and tell me where I'm getting things wrong, or what I've left out?
Ph Gh Kh Cheat Sheet
Bottom line: Fish prefer constant ph over perfect ph. Most fish will adapt a particular ph, but stress if ph swings, so most of the time, its better to keep ph steady, rather than keep it at a specific value.
Ph
Ph is a measure of how acidic or alkaline water is. A ph of 7 is neutral. Above 7 is alkaline, below 7 is acidic. The higher the value, the more alkaline, the lower, the more acidic.
This one is important:
The ph scale is logarithmic, which means that values on the scale ( 5...6….7...8...9...etc) don’t represent equal amounts of change in acidity or alkalinity. In other words, the difference between ph5 and ph6 is not the same as the difference in ph6 and ph7.
It’s not like a ruler, where the distance between 5cm and 6cm = 1cm and the distance between 6cm and 7cm also = 1cm. The ph scale moves outward from 7, with each “step” in the scale being ten times the size of the previous step. The scale below gives you an idea, but to be accurate, I’d have to fit 100 dashes in between 4 and 5, and between 9 and 10. The page is not wide enough
4-----------------------------------5 ---------- 6 - 7- 8 ---------- 9-----------------------------------10
In other words, if water changes from ph7 to ph6, it’s one step more acidic. But if it changes from ph6 to ph5, it’s ten steps more acidic. And if it changes from ph5 to ph4, it’s one hundred steps more acidic.
This is important, because fish are so sensitive to changes in ph. Remember that if the ph swings from ph8 to ph9, that’s going to be a big change from the fish’s point of view, while a change from ph7 to ph8 is not as big.
Ph and Ammonia Toxicity
Ammonia is toxic to fish. But it’s a lot more toxic in water with a high Ph, than in water with a low Ph.
Ph and Kh- “Buffering”
Kh stands for “Carbon Hardness” and measures how much Calcium Carbonate the water contains.
The Kh and Ph measures are related. The higher the Kh, the higher the Ph. Although Kh is not the only thing that raises Ph, it’s one of them
Kh is also referred to as the water’s “buffering ability”. If the water has a good “buffer” it means that it is less likely to change its Ph value. In other words, if the water in your tank has a low Kh value, this means that various events (cycling the tank, adding rocks or driftwood etc) are likely to make that Ph value change quickly. If the tank has a high Kh value, apart from the fact that the Ph will be higher too, whatever happens in that tank, it’s unlikely that the Ph will change much. High Kh = more stable Ph.
If you needed, for some reason, to lower the Ph of the tank, you’d have to lower the Kh as well. Not just because Kh increases the alkalinity, but also because Kh makes it harder to change the Ph.
This is one of the reasons it’s not a great idea to use distilled water in a fishtank. Distilled water sounds great - it’s pure, right? Its Ph is neutral (7) and it has a Kh of 0. No Calcium Carbonate at all in distilled water. That means that a tank full of distilled water can change it’s Ph easily and rapidly, a very bad idea for the fish.
Other reasons Kh is important
Calcium Carbonate (the value that is measured by Kh) is also a very important to the health of your fish, plants and invertebrates. It’s needed for bone and teeth development, it helps regulate blood flow, helps the immune system and metabolism, and also promotes healthy heart.
This is another reason why distilled water is not the best for fish. It contains none of the essential minerals like Calcium Carbonate.
Gh
Gh measures General hardness. It measures the amount of all the minerals in the tank, including Calcium Carbonate. So you can see that Kh and Gh are related. Gh measures Magnesium, Calcium Carbonate, Sodium, Sulphur, Potassium, and Chloride. But of these, the mineral that is most significant in the Gh measure is Magnesium. Magnesium is just as important for fish health as Calcium Carbonate. It aids digestion, immunity, growth, gill development among other things.
Common reasons Ph might change in an aquarium
Cycling, overstocking, lack of water changes
Nitrifying bacteria, the beneficial bacteria we rely on to process ammonia and nitrites into nitrates, use Calcium Carbonate in order to duplicate themselves and process their food (ammonia and nitrites).
If the Kh level in the water is low already, that means that the bacteria will use up a large percentage of the available Calcium Carbonate. Remember Kh is related to Ph and also buffers Ph? So if Kh drops, this means that Ph levels will drop as well, and because of the reduced buffering capacity, that drop might be fast. Some call this a “Ph Crash”.
This is especially likely to happen in the first stages of cycling a tank, when the bacteria is multiplying at a high rate. This can also happen if a tank is overstocked and there is a high level of ammonia present for the bacteria to process.
The drop in Ph as a result of the action of the nitrifying bacteria means that most tanks will gradually get more and more acidic. If a tank is neglected, with few or no water changes performed, the result might be a significant drop in Ph over time. This drop is likely to be fairly gradual and in itself might not be that bad for the fish. But if a big water change is performed at this point, the clean water coming in is likely to be quite different in Ph from the old water going out, and this can be stressful for the fish. In this situation it’s probably better to perform a series of small water changes over a number of days, rather than one big one all at once.
Ph and Bacteria
As mentioned in the previous section on Ph in cycling, the nitrifying bacteria need a certain level of Kh to duplicate and process the wastes in the water. Below a certain Ph, they can no longer do so. At or below a Ph or 6, nitrifying bacteria stop reproducing. This is why the nitrogen cycle might slow or stall in a tank where the Ph has crashed to 6 or below.
Carbon Dioxide CO2
When Carbon Dioxide comes in contact with water it dissolves and forms a weak acid called Carbonic Acid. So the more carbon dioxide there is in the water, the lower the Ph will be.
Carbon Dioxide can be absorbed from the atmosphere, be given off by plants, or added by a CO2 injection system.
Plants absorb CO2 during photosynthesis, when they are exposed to light, and the give off CO2 as part of their respiration process, when they are not exposed to light. This can mean that in a planted tank, the CO2 levels, and so the Ph, will swing gradually up and down during the day and night, as plants respond to light levels.
Chlorine
Chlorine is alkaline. It also outgasses out of water gradually, usually over a period of 24 hours. This is why, when measuring the Ph of your tapwater, it’s best to let it sit for 24 hours until the chlorine is gone, otherwise you will get an inaccurately high reading. Remember that water conditioners like Prime don’t remove the chlorine, it only makes it safe. So testing tap-water which has just had Prime added to it, will also give you an inaccurate Ph as the chlorine is still present.
Rocks, shells, Peat, Driftwood, and dead leaves.
Rocks that are high in calcium, like limestone or marble, and objects like sea-shells will leach calcium into the water and increase the Kh, and so also the Ph.
Driftwood, and dead leaves release tannin, which is acidic. So these will lower the Ph.
Peat bonds with the calcium carbonate in water, and prevents it from bonding with the water itself, so lowering the concentration of calcium carbonate in the water, resulting in a lower Kh.
“Soft” and “Hard” water.
The Gh value is usually the one people refer to when they talk about their water being “soft” or “hard”. Water with a low Gh is soft, water with a high Gh is hard. For example, areas with lots of limestone (high in calcium) will likely have very hard water, and typically, that calcium will leave stains and calcium deposits as the water evaporates.
Rain water is typically quite soft, it doesn’t have a high mineral content.
Remember that Gh and Kh are related (Gh includes the amount of Calcium Carbonate, or Kh) so if water has a low Gh, it has a low Kh as well. This means that soft water is likely to have very little buffering capacity, and the Ph of soft water is likely to change more easily, the Ph is more likely to be unstable.
Methods to change Kh and Gh
As mentioned, to lower Ph, it’s also necessary to lower Kh. Partly because Calcium Carbonate is alkaline, so removing it will lower the Ph, but also because the presence of Calcium Carbonate makes it more difficult to lower the Ph.
Methods to lower Kh include:
Adding CO2
Peat - it bonds with the Calcium Carbonate and prevents it from bonding with the water - thus lowering the Kh level in the water
Mixing in Reverse Osmosis water (RO water has 0 kh)
Methods to increase Kh include:
Adding baking soda (may not be a great idea as it changes levels rapidly, and you need to keep adding it over time)
Adding an airstone to increase turbulence and drive off CO2
Coral chips or limestone will leach Calcium into the water
Here are some of the articles I got this information from:
Important: Understanding PH, KH, GH In Home Aqauriums - 113548
Water Chemistry: pH, GH and KH What are they all? - Scott Thomson
KH - The Free Freshwater and Saltwater Aquarium Encyclopedia Anyone Can Edit - The Aquarium Wiki
Ammonia Toxicity and pH Changes
Ph Gh Kh Cheat Sheet
Bottom line: Fish prefer constant ph over perfect ph. Most fish will adapt a particular ph, but stress if ph swings, so most of the time, its better to keep ph steady, rather than keep it at a specific value.
Ph
Ph is a measure of how acidic or alkaline water is. A ph of 7 is neutral. Above 7 is alkaline, below 7 is acidic. The higher the value, the more alkaline, the lower, the more acidic.
This one is important:
The ph scale is logarithmic, which means that values on the scale ( 5...6….7...8...9...etc) don’t represent equal amounts of change in acidity or alkalinity. In other words, the difference between ph5 and ph6 is not the same as the difference in ph6 and ph7.
It’s not like a ruler, where the distance between 5cm and 6cm = 1cm and the distance between 6cm and 7cm also = 1cm. The ph scale moves outward from 7, with each “step” in the scale being ten times the size of the previous step. The scale below gives you an idea, but to be accurate, I’d have to fit 100 dashes in between 4 and 5, and between 9 and 10. The page is not wide enough
4-----------------------------------5 ---------- 6 - 7- 8 ---------- 9-----------------------------------10
In other words, if water changes from ph7 to ph6, it’s one step more acidic. But if it changes from ph6 to ph5, it’s ten steps more acidic. And if it changes from ph5 to ph4, it’s one hundred steps more acidic.
This is important, because fish are so sensitive to changes in ph. Remember that if the ph swings from ph8 to ph9, that’s going to be a big change from the fish’s point of view, while a change from ph7 to ph8 is not as big.
Ph and Ammonia Toxicity
Ammonia is toxic to fish. But it’s a lot more toxic in water with a high Ph, than in water with a low Ph.
Ph and Kh- “Buffering”
Kh stands for “Carbon Hardness” and measures how much Calcium Carbonate the water contains.
The Kh and Ph measures are related. The higher the Kh, the higher the Ph. Although Kh is not the only thing that raises Ph, it’s one of them
Kh is also referred to as the water’s “buffering ability”. If the water has a good “buffer” it means that it is less likely to change its Ph value. In other words, if the water in your tank has a low Kh value, this means that various events (cycling the tank, adding rocks or driftwood etc) are likely to make that Ph value change quickly. If the tank has a high Kh value, apart from the fact that the Ph will be higher too, whatever happens in that tank, it’s unlikely that the Ph will change much. High Kh = more stable Ph.
If you needed, for some reason, to lower the Ph of the tank, you’d have to lower the Kh as well. Not just because Kh increases the alkalinity, but also because Kh makes it harder to change the Ph.
This is one of the reasons it’s not a great idea to use distilled water in a fishtank. Distilled water sounds great - it’s pure, right? Its Ph is neutral (7) and it has a Kh of 0. No Calcium Carbonate at all in distilled water. That means that a tank full of distilled water can change it’s Ph easily and rapidly, a very bad idea for the fish.
Other reasons Kh is important
Calcium Carbonate (the value that is measured by Kh) is also a very important to the health of your fish, plants and invertebrates. It’s needed for bone and teeth development, it helps regulate blood flow, helps the immune system and metabolism, and also promotes healthy heart.
This is another reason why distilled water is not the best for fish. It contains none of the essential minerals like Calcium Carbonate.
Gh
Gh measures General hardness. It measures the amount of all the minerals in the tank, including Calcium Carbonate. So you can see that Kh and Gh are related
. Gh measures Magnesium, Calcium Carbonate, Sodium, Sulphur, Potassium, and Chloride. But of these, the mineral that is most significant in the Gh measure is Magnesium. Magnesium is just as important for fish health as Calcium Carbonate. It aids digestion, immunity, growth, gill development among other things.Common reasons Ph might change in an aquarium
Cycling, overstocking, lack of water changes
Nitrifying bacteria, the beneficial bacteria we rely on to process ammonia and nitrites into nitrates, use Calcium Carbonate in order to duplicate themselves and process their food (ammonia and nitrites).
If the Kh level in the water is low already, that means that the bacteria will use up a large percentage of the available Calcium Carbonate. Remember Kh is related to Ph and also buffers Ph? So if Kh drops, this means that Ph levels will drop as well, and because of the reduced buffering capacity, that drop might be fast. Some call this a “Ph Crash”.
This is especially likely to happen in the first stages of cycling a tank, when the bacteria is multiplying at a high rate. This can also happen if a tank is overstocked and there is a high level of ammonia present for the bacteria to process.
The drop in Ph as a result of the action of the nitrifying bacteria means that most tanks will gradually get more and more acidic. If a tank is neglected, with few or no water changes performed, the result might be a significant drop in Ph over time. This drop is likely to be fairly gradual and in itself might not be that bad for the fish. But if a big water change is performed at this point, the clean water coming in is likely to be quite different in Ph from the old water going out, and this can be stressful for the fish. In this situation it’s probably better to perform a series of small water changes over a number of days, rather than one big one all at once.
Ph and Bacteria
As mentioned in the previous section on Ph in cycling, the nitrifying bacteria need a certain level of Kh to duplicate and process the wastes in the water. Below a certain Ph, they can no longer do so. At or below a Ph or 6, nitrifying bacteria stop reproducing. This is why the nitrogen cycle might slow or stall in a tank where the Ph has crashed to 6 or below.
Carbon Dioxide CO2
When Carbon Dioxide comes in contact with water it dissolves and forms a weak acid called Carbonic Acid. So the more carbon dioxide there is in the water, the lower the Ph will be.
Carbon Dioxide can be absorbed from the atmosphere, be given off by plants, or added by a CO2 injection system.
Plants absorb CO2 during photosynthesis, when they are exposed to light, and the give off CO2 as part of their respiration process, when they are not exposed to light. This can mean that in a planted tank, the CO2 levels, and so the Ph, will swing gradually up and down during the day and night, as plants respond to light levels.
Chlorine
Chlorine is alkaline. It also outgasses out of water gradually, usually over a period of 24 hours. This is why, when measuring the Ph of your tapwater, it’s best to let it sit for 24 hours until the chlorine is gone, otherwise you will get an inaccurately high reading. Remember that water conditioners like Prime don’t remove the chlorine, it only makes it safe. So testing tap-water which has just had Prime added to it, will also give you an inaccurate Ph as the chlorine is still present.
Rocks, shells, Peat, Driftwood, and dead leaves.
Rocks that are high in calcium, like limestone or marble, and objects like sea-shells will leach calcium into the water and increase the Kh, and so also the Ph.
Driftwood, and dead leaves release tannin, which is acidic. So these will lower the Ph.
Peat bonds with the calcium carbonate in water, and prevents it from bonding with the water itself, so lowering the concentration of calcium carbonate in the water, resulting in a lower Kh.
“Soft” and “Hard” water.
The Gh value is usually the one people refer to when they talk about their water being “soft” or “hard”. Water with a low Gh is soft, water with a high Gh is hard. For example, areas with lots of limestone (high in calcium) will likely have very hard water, and typically, that calcium will leave stains and calcium deposits as the water evaporates.
Rain water is typically quite soft, it doesn’t have a high mineral content.
Remember that Gh and Kh are related (Gh includes the amount of Calcium Carbonate, or Kh) so if water has a low Gh, it has a low Kh as well. This means that soft water is likely to have very little buffering capacity, and the Ph of soft water is likely to change more easily, the Ph is more likely to be unstable.
Methods to change Kh and Gh
As mentioned, to lower Ph, it’s also necessary to lower Kh. Partly because Calcium Carbonate is alkaline, so removing it will lower the Ph, but also because the presence of Calcium Carbonate makes it more difficult to lower the Ph.
Methods to lower Kh include:
Adding CO2
Peat - it bonds with the Calcium Carbonate and prevents it from bonding with the water - thus lowering the Kh level in the water
Mixing in Reverse Osmosis water (RO water has 0 kh)
Methods to increase Kh include:
Adding baking soda (may not be a great idea as it changes levels rapidly, and you need to keep adding it over time)
Adding an airstone to increase turbulence and drive off CO2
Coral chips or limestone will leach Calcium into the water
Here are some of the articles I got this information from:
Important: Understanding PH, KH, GH In Home Aqauriums - 113548
Water Chemistry: pH, GH and KH What are they all? - Scott Thomson
KH - The Free Freshwater and Saltwater Aquarium Encyclopedia Anyone Can Edit - The Aquarium Wiki
Ammonia Toxicity and pH Changes
