Aquarium GH, KH, PH; Chemistry, What to Know

Aquarium Chemistry, in particular GH, KH, & pH

Fish Chemistry for beginners, GH, KH, pH, MineralsThe purpose of this article is to provide basic answers to the most common questions asked about Aquarium Chemistry, in particular GH, KH, & pH

Much of this information is courtesy of:
“Aquarium Chemistry; Electrolytes, GH, KH, pH, More”
Please see the above referenced article for more in depth information.


  • What is the importance of GH, KH, pH?

    This is a question that really deserves a long answer for best understanding, but I will make it as short and simple as possible.

    *GH; is “general hardness” by the common long standing definition. A more accurate description for aquarium (or pond) needs is the calcium, magnesium, and other essential minerals found in an aquarium.
    More importantly a test for GH is simply measure of these elements presence and the ability of maintaining essential positive mineral ions, but not the guarantee these mineral ions are present.
    See: Aquarium Chemistry; GH

    Also reference Fish Beginner’s new article about fish/aquarium health and how mineral ions play a roll in long term fish health:
    What Makes for Healthy Fish in an Aquarium and What Does Not?

    *KH; is carbonate hardness, but a more accurate description is alkaline buffering which is what prevents a drop in pH when a certain volume of acid is introduced into an aquarium.
    See: Aquarium Chemistry; KH

    *pH; can be stated as aquarium water with a pH less than 7 is “acidic” while water with a pH greater than 7 is “alkaline”.
    See: Aquarium Chemistry; PH


  • What is the importance of testing for GH?

    GH is the storage spot for your essential mineral ions that are necessary for aquatic life, and bio chemistry. Without GH, or better “positive mineral ions”, your tank life will suffer over time.

  • What is the best GH for an aquarium?

    This is a loaded question as there is no “best” GH.
    Knowing your exact numbers can be important for advanced planted aquarium keepers.
    See: Planted Aquarium Care; Nutrients & Ferts (GH).

    However for the average aquarium keeper simply knowing you have at least SOME general hardness (GH) is what is most important and by some I would recommend generally at least 150 ppm.
    Often far too much is made of a GH of 400+ being too high when the reality of fish bio chemistry, this level of GH is rarely a problem.
    Often many will pass around old myths about a high GH being unsafe for certain fish such as Angelfish or Bettas, however both long term observations and more recent research, such as into Aquarium Redox has shown this myth to be just that; A MYTH.
    Please Reference: Aquarium Chemistry; Facts & Complete Information

    A way to look at this (borrowed in part from Aquarium Chemistry; GH) is to look at your GH as the lead oxide, sulfuric acid, & other materials that make up a car battery, without these materials you could not hold a positive charge. The same goes for GH as without the elements that make up GH, your aquarium cannot contain the essential electrolytes for fish health.

    Simply put, having a GH of 250 ppm does not necessarily mean you have these essential mineral ions, as with a car battery that is not charged. You have the materials in the battery, but they do you no good, unless charged.
    Same goes for the GH, you must first have the elements to make up the GH, then you can add the charge.

    Having a very low or non existent GH certainly means you cannot maintain these ESSENTIAL electrolytes/mineral ions that ALL fish require, including Bettas, Ram Cichlids, etc., just not as as high of amounts as fish such as Guppies, or Goldfish might require.
    As well, soft ions [Anions] such as sodium and potassium are critical in the tissues of fish for correct function of nervous and muscular tissues. Aquarium water that is has a low GH can cause these critical ions to leave the fish’s gills/tissues and cause a fish to go into osmotic stress.

    From: Aquarium Chemistry
    “The presence of free (ionic) calcium at relatively high concentrations in culture water helps reduce the loss of other salts (e.g. sodium and potassium) from fish body fluids (i.e. blood). Sodium and potassium are the most important salts in fish blood and are critical for normal heart, nerve and muscle function. In low calcium water, fish can lose (leak) substantial quantities of these salts into the water. Fish must then use energy supplied by their feed to re-absorb lost salts. That can reduce the energy available for growth and may extend the time necessary to grow fish to market size. For some species, environmental calcium is required for good survival”

    Failure to maintain these mineral ions can result in opportunistic infections such as Columnaris and/or difficulty in achieving a cure even with the best of treatment methods.
    See: Aquarium Fish Columnaris
    OR Columnaris in Fish, Bettas; What is Correct?

    Assuming properly mineralized water (from your tap or well), a water change can add in these ions.
    As well buffer/mineral supplements, AAP Wonder Shells, Cichlid Salts, and other products are ways to maintain these essential mineral ions.
    Tests into aquarium Redox has shown that the use of a dissolving mineral block such as a FRESH AAP Wonder Shell best maintains these positive mineral ions [Cations] as well as lowering oxidative stress in fish.
    As a side note, there are a lot of parasite sellers now for the Wonder Shell, please support the professional aquarium keeping hobby by purchasing ONLY from AAP as most all others are not fresh product without correct usage information too. Sadly many YouTubers without any real knowledge themselves, have purchased from these parasite sellers such as Amazon and then repeat the improper use information thus further perpetuating misinformation about their use often resulting in poor results!

    In summary, the term “too high GH” has been proven in most instances to be a myth and more importantly depriving your fish of essential mineral Cations can lower fish disease resistance and make treatment of sick fish more difficult due to oxidative stress!

    This video helps demonstrate how mineral Cations remove oxidizers (including chlorine):
    AAP Wonder Shell, Oxidizers, chlorine, Aquarium Co-op
    VIDEO: Aquarium Redox Part 2 | Oxidizers & Reducers | AAP Wonder Shell Experiment

    Further Reading:
    #Healthy Aquarium Redox to Lower Fish Oxidative Stress
    #Fish Diseases | How to Treat Sick Fish

    Our Recommended Product Resources:
    #Unique American Aquarium Wonder Shells This is the ONLY source for FRESH and unique complete AND Medicated Wonders, these cannot be found elsewhere!
    It is worthy of note that AAP Wonder Shells loose their positive mineral “ion charges” [Cations) just like a battery does, so clearance AAP Wonder Shells will not be fresh. Amazon in particular is NOT authorized to sell this product, thus they sell clearance product. So beware of websites pushing Amazon links for this product such as FishLab!

    #Cichlid Salt, Mineral Supplement from AAP

  • Aquarium Wonder Shells
    Video; How Long Do Wonder Shells Last?

  • What is the importance of testing for KH?

    Knowing your KH is is important for maintaining a stable pH, as a low or falling KH is often a predictor of an impending pH crash, which if rapid and over 1 on the logarithmic pH scale can be deadly to some fish.
    As well testing your KH on a regular basis such as once per week can help you determine if your aquarium needs cleaning or if your bio load is too high. This is due to normal acid production of the aquarium bio processes which includes the nitrogen cycle.

    It is noteworthy that many aquarium professionals regard the KH test as more important than a pH test as it tells much more about an aquariums health.

    See our other article:
    Aquarium Nitrogen Cycle, Fish Beginner

    You may need to add a KH (alkaline buffer) to your tap/well water prior to introduction to your aquarium or only for drops in your KH between water changes due to natural biological acid production.
    Your test kit or test strips will determine whether either or both modes of buffer use are necessary.
    As well the desired KH level/reading will determine the amount needed. For example a drop from 150 ppm to 125 ppm in a 20 gallon tank requires very little buffer. I generally recommend less than half the amount recommended by AAP/SeaChem Alkaline Buffer for KH adjustment assuming the pH is where you want it.

    Our Recommended Product Resource:
    #SeaChem Alkaline Buffer from AAP

  • What is the best KH for an aquarium?

    As with GH, there is not “Best” KH for a particular aquarium.
    As per this article:
    Aquarium Chemistry; Suggestions
    This “Best KH” can range from 50 to 250 ppm based on the fish kept [or even left for some Amazon River Biotopes].
    What is important is find a number (ppm) that works best and MAINTAINING THIS NUMBER before and after cleanings, etc.
    Maintaining this KH number when keep your pH from crashing and help with fluctuations in the bio load of the aquarium.

    As an example, if your pH is 7.0 and your KH is 80 ppm after a cleaning but a day or two later has fallen to 6.5; your aquarium likely needs a higher KH to maintain 7.0 AND/OR you may too much acid producing decomposition, peat, driftwood, etc.
    Reducing the acid production and/or raising KH ppm is called for.
    The reverse can be said for an aquarium that bounces up in pH after a water change.

  • What is the importance of testing for PH?

    Generally, despite popular opinion, KH is more important a regular test than pH since it predicts pH instability.
    This said, test for pH is important before and after water changes, this includes the new water to be added after a water change so as to prevent pH shock by adding water to an aquarium that could change the aquarium pH more than .5 either direction on the logarithmic pH scale.
    Reference: Aquarium, Reasons and Methods for Water Changes

    Many planted aquarium keepers, often run air stones at night to equalize oxygen and COs since plants cease production of O2, HOWEVER it is worthy of note that an air stone can drive off enough CO2, that the pH can climb considerably at night.
    So it is important to test pH evening and morning and adjust your air stone accordingly to prevent to drastic a pH swing.
    BTW, the reason for this is CO2 in the water can produce carbonic acid, that will naturally lower pH, but when driven off by nightly increases in aeration, this can result in pH swings, generally in smaller aquaria.

    As well, pH should be checked when problems occur in your aquarium such as fish sick, then compare this pH to past tests (which it is best to record all aquarium tests in a journal).

  • What is the best PH for an aquarium?

    Again, there is NO one best pH number!
    PH in fact is a parameter that is often over-rated as per an exact number.
    Since the pH scale is logarithmic*, the more important consideration is avoiding a sudden change of more than .5 in a day or less.
    Even with fish such as Discus which come from water of quite low pH water, they will adapt with proper acclimation to waters as high as 7.5 and even breed in this water.

    The bottom line unlike many other aquarium parameters, is to not worry too much about specific pH, rather maintain a STABLE pH!
    A stable pH is vastly more important to most fish including Discus and Bettas than the actual number (it is stable KH you need to aim for).
    In consulting with experts, that have kept/bred Angels, Discus, etc in a pH as high as 7.5, but where they have seen many issues when consulting with fish keepers having “fish issues”, often “chasing” a specific pH number was their problem.

    As well, pH should be checked when problems occur in your aquarium such as fish sick, then compare this pH to past tests (which it is best to record all aquarium tests in a journal).

    *Logarithmic= meaning a change of pH from say 7.0 to 6.0 is a ten fold increase in acidity, while a change from 7.0 to 5.0 is 100 times change in acidity (it also goes the other direction as to alkalinity).

  • Since the Bio Load has been referred to in this article, What is the bio load?

    The bio load (more correctly: the biological load) is the nitrogen processing demand in an aquarium system (filters, gravel, plants, etc) due to organics in the aquarium.
    These organics include the fish and other inhabitants, uneaten fish food, dead or decomposing fish or other now deceased inhabitants, or decaying plant material

  • Can I use Crushed Coral to raise KH in my Aquarium?

    Since crushed coral does not have many carbonates or bi-carbonates readily available without a chemical reaction, the answer is mostly no since it is carbonates that you are measuring when you test your KH.
    This noted, aragonites are much more “refined” than crushed coral if you will, and allow for more surface area for contact of acidic water to allow for the breakdown of its components, thereby releasing more of the carbonates and bi-carbonates and raising KH.

    Even so, for true KH and low pH issues, stronger and more dramatic KH buffer such as SeaChem Alkaline Buffer or Cichlid salt are generally called for since aragonite is still better at maintaining KH that addressing serious KH issues.

  • What can I do to maintain a desired KH and pH without too much fluctuation including after water changes?

    Adjusting your new water whether it be tap, well, RO, or a blend of RO & tap or well prior to introduction can save your aquarium from many a roller coaster ride in pH and KH numbers after a water change.
    Using products such as AAP Alkaline or Acid Buffer in your new water to a number that equals what you want your aquarium to be is a simple way to manage this.

    For ongoing aquarium maintenance of pH/KH; regular use of these products in whatever amount is needed to “do the job” can help with this as well.
    For keeping pH & KH lower, use of Driftwood and Pillow Moss can help naturally keep lower pH/KH numbers.

    Our Recommended Product Resources:
    #AAP Alkaline Buffer
    #Malawi Buffer from AAP
    #AAP Acid Buffer
    #Pillow Moss from AAP
    #Select Aquarium Driftwood

  • Can I use water from a Home Water Softener in my Aquarium?

    The simple answer is absolutely NOT!

    While some persons claim they have done this without a problem, I can also point out that these persons are not running a controlled test. Experts in the aquarium service business have reported many times vastly improved fish health when they discovered this was a problem with new clients.

    More importantly bio-chemistry science PROVES this is not healthy for ANY fish (including Discus).
    The use of soft water from sources that utilize sodium or even from aquarium conditioners that have sodium bases is that the sodium often drives out the essential mineral cations. The result is nearly NON-existent ESSENTIAL calcium, magnesium, and other positive mineral ions. This can have severe affects on all fish, but is an especially noteworthy problem in Goldfish, Livebearers, and Rift Lake Cichlids

    The above is a quote from:
    Softened Water; Home/Office Water Softeners Use

    Adding back minerals such as in the form of Wonder Shells, Replenish, or Equilibrium will generally be futile as long as the sodium levels are high, as the minerals will continue to be precipitated out of your aquarium.

  • Does the health of of your Aquarium Bio Filter affect aquarium chemistry parameters such as pH and KH?

    The simple answer is most definitely YES!
    As this article has alluded to already as per KH/pH, often bounces both up or down in these parameters are much more common in small tanks where a healthy bio filter for maintaining the aquariums nitrogen cycle is difficult and often not 100% possible.

    This can also be a problem in any size aquaria where the bio filter is either unestablished or weak, often resulting in ammonia and nitrite spikes.

    See our other article for a further explanation:
    Aquarium Nitrogen Cycle, Fish Beginner

#Freshwater Aquarium Care, Basics
#Fish Diseases | How to Treat Sick Fish

Healty Freshwater Aquarium Care
What Makes for Healthy Fish in an Aquarium and What Does Not?

Treatment and prevention of aquarium fish disease
My Fish are Sick, What do I do?

#SeaChem Products
#AAP Professional/Aquatronics Aquarium Treatments/Products

#Lignite Aquarium Carbon
#Red Pest, Invented Fish Disease, Actually Septicemia
#API/ AAP Furan 2
#Helpful Aquarium & Pond Keeping Acronyms, Abbreviations


Columnaris in Fish VideoYouTube; How to: 4 Steps Columnaris Treatment Fish Bacterial Infection
This video goes over the basics of the full four step plan of properly treating Columnaris in aquarium fish


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