Practical Water Chemistry

livefish Administrator Posts: 278
edited December 2007 in Beginner's Forum
What You Need to Know About Water Chemistry, and Why
Water in nature is rarely pure in the "distilled water" sense; it contains dissolved salts, buffers, nutrients, etc., with exact concentrations dependent on local conditions. Fish (and plants) have evolved over millions of years to the specific water conditions in their native habitats and may be unable to survive in significantly different environments.

Beginners (especially the lazy) should take the easy approach of selecting fish whose needs match the qualities of their normal tap water. Alternatively, an advanced (and energetic!) aquarist can change the water characteristics to match the fish's needs, though doing so is almost always more difficult than first appears. In either case, you need to know enough about water chemistry to ensure that the water in your tank has the right properties for the fish you are keeping.

Water has four measurable properties that are commonly used to characterize its chemistry. They are pH, buffering capacity, general hardness and salinity. In addition, there are several nutrients and trace elements.

pH refers to water being either an acid, base, or neither (neutral). A pH of 7 is said to be neutral, a pH below 7 is "acidic" and a pH above 7 is "basic" or "alkaline". Like the Richter scale used to measure earthquakes, the pH scale is logarithmic. A pH of 5.5 is 10 times more acidic than water at a pH of 6.5. Thus, changing the pH by a small amount (suddenly) is more of a chemical change (and more stressful to fish!) than might first appear.

To a fishkeeper, two aspects of pH are important. First, rapid changes in pH are stressful to fish and should be avoided. Changing the pH by more than .3 units per day is known to stress fish. Thus, you want the pH of your tank to remain constant and stable over the long haul. Second, fish have adapted to thrive in a (sometimes narrow) pH range. You want to be sure that your tank's pH matches the specific requirements of the fish you are keeping.

Most fish can adjust to a pH somewhat outside of their optimal range. If your water's pH is naturally within the range of 6.5 to 7.5, you will be able to keep most species of fish without any problems. If your pH lies within this range, there is probably no need to adjust it upward or downward.

Buffering Capacity (KH, Alkalinity)
Buffering capacity refers to water's ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined with one another; although one might think that adding equal volumes of an acid and neutral water would result in a pH halfway in between, this rarely happens in practice. If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. Conceptually, a buffer acts somewhat like a large sponge. As more acid is added, the "sponge" absorbs the acid without changing the pH much. The "sponge's" capacity is limited however; once the buffering capacity is used up, the pH changes more rapidly as acids are added.

Buffering has both positive and negative consequences. On the plus side, the nitrogen cycle produces nitric acid (nitrate). Without buffering, your tank's pH would drop over time (a bad thing). With sufficient buffering, the pH stays stable (a good thing). On the negative side, hard tap water often almost always has a large buffering capacity. If the pH of the water is too high for your fish, the buffering capacity makes it difficult to lower the pH to a more appropriate value. Naive attempts to change the pH of water usually fail because buffering effects are ignored.

In freshwater aquariums, most of water's buffering capacity is due to carbonates and bicarbonates. Thus, the terms "carbonate hardness" (KH), "alkalinity" and "buffering capacity" are used interchangeably. Although technically not the same things, they are equivalent in practice in the context of fishkeeping. Note: the term "alkalinity" should not be confused with the term "alkaline". Alkalinity refers to buffering, while alkaline refers to a solution that is a base (i.e., pH > 7).

How much buffering does your tank need? Most aquarium buffering capacity test kits actually measure KH. The larger the KH, the more resistant to pH changes your water will be. A tank's KH should be high enough to prevent large pH swings in your tank over time. If your KH is below roughly 4.5 dH, you should pay special attention to your tank's pH (e.g, test weekly, until you get a feel for how stable the pH is). This is ESPECIALLY important if you neglect to do frequent partial water changes. In particular, the nitrogen cycle creates a tendency for an established tank's pH to decrease over time. The exact amount of pH change depends on the quantity and rate of nitrates produced, as well as the KH. If your pH drops more than roughly two tenths of a point over a month, you should consider increasing the KH or performing partial water changes more frequently. KH doesn't affect fish directly, so there is no need to match fish species to a particular KH.

Note: it is not a good idea to use distilled water in your tank. By definition, distilled water has essentially no KH. That means that adding even a little bit of acid will change the pH significantly (stressing fish). Because of its instability, distilled (or any essentially pure water) is never used directly. Tap water or other salts must first be added to it in order to increase its GH and KH.

General Hardness (GH)
General hardness (GH) refers to the dissolved concentration of magnesium and calcium ions. When fish are said to prefer "soft" or "hard" water, it is GH (not KH) that is being referred to.

Note: GH, KH and pH form the Bermuda's Triangle of water chemistry. Although the three properties are distinct, they all interact with each other to varying degrees, making it difficult to adjust one without impacting the other. That is one reason why beginning aquarists are advised NOT to tamper with these parameters unless absolutely necessary. As an example, "hard" water frequently often comes from limestone aquifers. Limestone contains calcium carbonate, which when dissolved in water increases both the GH (from calcium) and KH (from carbonate) components. Increasing the KH component also usually increases pH as well. Conceptually, the KH acts as a "sponge" absorbing the acid present in the water, raising the water's pH.

Water hardness follows the following guidelines. The unit dH means "degree hardness", while ppm means "parts per million", which is roughly equivalent to mg/L in water. 1 unit dH equals 17.8 ppm CaCO3. Most test kits give the hardness in units of CaCO3; this means the hardness is equivalent to that much CaCO3 in water but does not mean it actually came from CaCO3.

General Hardness

0 - 4 dH, 0 - 70 ppm : very soft
4 - 8 dH, 70 - 140 ppm : soft
8 - 12 dH, 140 - 210 ppm : medium hard
12 - 18 dH, 210 - 320 ppm : fairly hard
18 - 30 dH, 320 - 530 ppm : hard
higher : liquid rock (Lake Malawi and Los Angeles, CA)

Salinity refers to the total amount of dissolved substances. Salinity measurements count both GH and KH components as well as such other substances as sodium. Knowing water's salinity becomes important in salt water aquariums. In freshwater tanks, knowing pH, GH and KH suffices.

Salinity is usually expressed in terms of its specific gravity, the ratio of a solution's weight to weight of an equal volume of distilled water. Because water expands when heated (changing its density), a common reference temperature of 59F degrees is used. Salinity is measured with a hydrometer, which is calibrated for use at a specific temperature (e.g., 75F degrees is common).

One component of salinity that neither GH or KH includes is sodium. Some freshwater fish tolerate (or even prefer) a small amount of salt (it stimulates slime coat growth). Moreover, parasites (e.g., ick) do not tolerate salt at all. Thus, salt in concentrations of (up to) 1 tablespoon per 5 gallons can actually help prevent and cure ick and other parasitic infections.

On the other hand, some species of fish do not tolerate ANY salt well. Scaleless fish (in general) and some Corydoras catfish are far more sensitive to salt than most freshwater fish. Add salt only if you are certain that all of your tank's inhabitants prefer it or can at least tolerate it.

Nutrients and Trace Elements
In addition to GH, KH, pH and salinity, there are a few other substances you may want to know about. Most tap water contains an assortment of nutrients and trace elements in very low concentrations. The presence (or absence) of trace elements can be important in some situations, specifically:

* nitrates, which are discussed in great length in this FAQ in conjunction with the NITROGEN CYCLE;
* phosphates, the second most prominent nutrient. Phosphates have been linked to algae growth. If you have persistent algae problems, high phosphates may be a contributing factor. In a plant tank, ideal phosphate levels are .2 mg/L or lower. To control algae, frequent partial water changes are often recommended to reduce nutrient levels. If your tap water contains excess phosphate, water changes may be aggravating the situation. Your local water company can tell you what the exact phosphate levels are.
* iron, manganese and other trace elements. Plants need iron in trace quantities to grow. Tap water in many areas contains no iron at all. Consult the PLANT FAQ for more details.

Altering Your Water's Chemistry
Hardening Your Water (Raising GH and/or KH)
The following measurements are approximate; use a test kit to verify you've achieved the intended results. Note that if your water is extremely soft to begin with (1 degree KH or less), you may get a drastic change in pH as the buffer is added.

To raise both GH and KH simultaneously, add calcium carbonate (CaCO3). 1/2 teaspoon per 100 liters of water will increase both the KH and GH by about 1-2 dH. Alternatively, add some sea shells, coral, limestone, marble chips, etc. to your filter.

To raise the KH without raising the GH, add sodium bicarbonate (NaHCO3), commonly known as baking soda. 1/2 teaspoon per 100 Liters raises the KH by about 1 dH. Sodium bicarbonate drives the pH towards an equilibrium value of 8.2.

Raising and Lowering pH
One can raise or lower pH by adding chemicals. Because of buffering, however, the process is difficult to get right. Increasing or decreasing the pH (in a stable way) actually involves changing the KH. The most common approach is to add a buffer (in the previous section) whose equilibrium holds the pH at the desired value.

Muriatic (hydrochloric) acid can be used to reduce pH. Note that the exact quantity needed depends on the water's buffering capacity. In effect, you add enough acid to use up all the buffering capacity. Once this has been done, decreasing the pH is easy. However, it should be noted that the resultant lower-pH water has much less KH buffering than it did before, making it more susceptible to pH swings when (for instance) nitrate levels rise. Warning: It goes without saying that acids are VERY dangerous! Do not use this approach unless you know what you are doing, and you should treat the water BEFORE adding it to the aquarium.

Products such as "pH-Down" are often based on a phosphoric acid buffer. Phosphoric acid tends to keep the pH at roughly 6.5, depending on how much you use. Unfortunately, use of phosphoric acid has the BIG side effect of raising the phosphate level in your tank, stimulating algae growth. It is difficult to control algae growth in a tank with elevated phosphate levels. The only advantage over hydrochloric acid is that pH will be somewhat better buffered at its lower value.

One safe way to lower pH WITHOUT adjusting KH is to bubble CO2 (carbon dioxide) through the tank. The CO2 dissolves in water, and some of it forms carbonic acid. The formation of acid lowers the pH. Of course, in order for this approach to be practical, a steady source of CO2 bubbles (e.g. a CO2 tank) is needed to hold the pH in place. As soon as the CO2 is gone, the pH bounces back to its previous value. The high cost of a CO2 injection system precludes its use as a pH lowering technique in most aquariums (though see the PLANT FAQ for inexpensive do-it-yourself alternatives). CO2 injection systems are highly popular in heavily-planted tanks, because the additional CO2 stimulates plant growth.

Softening Your Water (i.e., lowering GH)
Some fish (e.g., discus, cardinal tetras, etc.) prefer soft water. Although they can survive in harder water, they are unlikely to breed in it. Thus, you may feel compelled to soften your water despite the hassle involved in doing so.

Typical home water softeners soften water using a technique known as "ion exchange". That is, they remove calcium and magnesium ions by replacing them with sodium ions. Although this does technically make water softer, most fish won't notice the difference. That is, fish that prefer soft water don't like sodium either, and for them such water softeners don't help at all. Thus, home water softeners are not an appropriate way to soften water for aquarium use.

Fish stores also market "water softening pillows". They use the same ion-exchange principle. One "recharges" the pillow by soaking it in a salt water solution, then places it in the tank where the sodium ions are released into the water and replaced by calcium and magnesium ions. After a few hours or days, the pillow (along with the calcium and magnesium) are removed, and the pillow recharged. The pillows sold in stores are too small to work well in practice, and shouldn't be used for the same reason cited above.

Peat moss softens water and reduces its hardness (GH). The most effective way to soften water via peat is to aerate water for 1-2 weeks in a bucket containing peat moss. For example, get a (plastic) bucket of the appropriate size. Then, get a large quantity of peat (a gallon or more), boil it (so that it sinks), stuff it in a pillow case, and place it in the water bucket. Use an air pump to aerate it. In 1-2 weeks, the water will be softer and more acidic. Use this aged water when making partial water changes on your tank.

Peat can be bought at pet shops, but it is expensive. It is much more cost-effective to buy it in bulk at a local gardening shop. Read labels carefully! You don't want to use peat containing fertilizers or other additives.

Although some folks place peat in the filters of their tanks, the technique has a number of drawbacks. First, peat clogs easily, so adding peat isn't always effective. Second, peat can be messy and may cloud the water in your tank. Third, the exact quantity of peat needed to effectively soften your water is difficult to estimate. Using the wrong amount results in the wrong water chemistry. Finally, when doing water changes, your tank's chemistry changes when new water is added (it has the wrong properties). Over the next few days, the chemistry changes as the peat takes effect. Using aged water helps ensure that the chemistry of your tank doesn't fluctuate while doing water changes.

Hard water can also be softened by diluting it with distilled water or R/O water. R/O (reverse-osmosis) water is purified water made by a R/O unit. Unfortunately, R/O units are too expensive ($100-$500) for most hobbyists. R/O water can also be purchased at some fish stores, but for most folks the expense and hassle are not worth it. The same applies to distilled water purchased at grocery stores.

These FAQs owe their existence to the contributors of the net, and as such it belongs to the readers of rec.aquaria and alt.aquaria. Articles with attributions are copyrighted by their original authors. Copies of the FAQs can be made freely, as long as it is distributed at no charge, and the disclaimers and the copyright notice are included.


  • Wizzbone
    Wizzbone Member Posts: 30
    HI I read your Water chemistry and I thank you for the information.

    However, I have been trying to find out the exact chemistry formulae, to show the inter-reaction of PH KH GH NH3 etc...

    For example CO2 and (rain) water causes Acid rain.. So CO2 is Acid causing how? (just make up: CO2 + H2O = CH2O3 = Acid? ) This formala I just made up, but you get the idea.

    So if NH3 breaks down to NO2 this this has as byproduct more O2 or CO2 or whatever that influences either the PH or KH? or does this create Aciditiy or abosorbs calcium?... Then from NO2 to NO3 what happens?

    You said:
    "the nitrogen cycle creates a tendency for an established tank's pH to decrease over time. The exact amount of pH change depends on the quantity and rate of nitrates produced, as well as the KH." I realised this fro my measurements of my tank. But How? what is the formulae (make up: NH3 +H20 = NO2 + ??, or NH3 + CaCO3 = NO2 + H20 +CaCH) (What is CaCH?)

    You said: "The unit dH means "degree hardness", while ppm means "parts per million", which is roughly equivalent to mg/L in water. 1 unit dH equals 17.8 ppm CaCO3. Most test kits give the hardness in units of CaCO3; this means the hardness is equivalent to that much CaCO3 in water but does not mean it actually came from CaCO3. "

    Ok what IS in the water that affect the KH reading? How does that changes over time? Does this involve CO2 or H2O or whatever?

    You said:
    "To raise both GH and KH simultaneously, add calcium carbonate (CaCO3). 1/2 teaspoon per 100 liters of water will increase both the KH and GH by about 1-2 dH. Alternatively, add some sea shells, coral, limestone, marble chips, etc. to your filter." If the dH is NOT from CaCO3, then why add this?

    To raise the KH without raising the GH, add sodium bicarbonate (NaHCO3), commonly known as baking soda. 1/2 teaspoon per 100 Liters raises the KH by about 1 dH. Sodium bicarbonate drives the pH towards an equilibrium value of 8.2. If the KH gets less (currently in my tank gone down from KH4 to KH2) How does that work? NH3 + NaHCO3 = ??

    Nevermind you get the gist of what I want to know....
  • Ken Boorman
    Ken Boorman Member Posts: 224
    H2O + CO2 = H2CO (carbonic acid) Wikipedia explains it better than I can. It's been too long (35 years) since I left school <!-- s:D --><img src="{SMILIES_PATH}/icon_biggrin.gif" alt=":D" title="Very Happy" /><!-- s:D -->

    Now, An acid is a compound which donates a Hydrogen ion (H+) to another compound. This effectively reduces the pH because it has a hydrogen ion activity which is greater than pure water.

    As for the rest.... what are you trying to do to your water? Make it more acid, or more alkaline? TDS (Total Dissolved Solids) plays a major part in keeping fish healthy too. I am convinced that a lot of what we call pH shock is actually caused by vastly different TDS rates in water the fish are being kept in/moved to. It effects them because the osmotic pressure variance makes the body react weirdly at times.

  • Wizzbone
    Wizzbone Member Posts: 30
    Found it!

    This is what I am looking for: It is not the whole picture but at least some is clear:

    NH4+(ammonia) + 2 H2O(water) >>> NO2-(nitrite) + 8 H+(hydrogen ions)
    NO2-(nitrite) + H2O(water) >>> NO3- (nitrate) + 2 H+(hydrogen ions)

    "Pure neutral water contains the same amount of H+ and OH- (hydroxide). The amounts are than equal to 10-7 moles per liter. A mule H+ is 6.022x1023 H+ atoms. Most, "all", of the H+ in water binds with water molecules and form H3O+ and it technically the amount of H3O+ molecules in the water that decides it's pH. The more H3O+ molecules the lower the pH."

    So H+ ions bind with H20 to make H30, this in turn affects the PH!

    NO how do PH and KH and GH affect each other??

    Some info I found: (isn't this exciting? I tried to explain to my wife aboutthe chemistry here, but she gets this far-away look lol)

    "General Hardness or GH is the measure of calcium (Ca++) and magnesium ions (Mg++) dissolved in water.
    Carbonate Hardness, also known as KH, refers to the concentration of bicarbonate (HCO3-) and carbonate (CO3--) dissolved in water. "

    "Calcium and magnesium carry a positive charge and form "ion pairs" with negatively charged ions like bicarbonate, forming calcium bicarbonate and magnesium bicarbonate; Ca(HCO3)2 & Mg(HCO3)2."

    OK.... now how does that fit with these H+ and the other items?

    Show you guys a chart I copy/paste from a website somewhere:

    \ pH | 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 8.0
    KH\ |
    0.5 | 15 9.3 5.9 3.7 2.4 1.5 0.9 0.6 0.2
    1.0 | 30 19 12 7 5 3 1.9 1.2 0.3
    1.5 | 44 28 18 11 7 4 2.8 1.8 0.4
    2.0 | 59 37 24 15 9 6 4 2.4 0.6
    2.5 | 73 46 30 19 12 7 5 3 0.7
    3.0 | 87 56 35 22 14 9 6 4 0.9
    3.5 | 103 65 41 26 16 10 7 4 1.0
    4.0 | 118 75 47 30 19 12 6 5 1.2
    5.0 | 147 93 59 37 23 15 9 6 1.5
    6.0 | 177 112 71 45 28 18 11 7 1.8
    8.0 | 240 149 94 59 37 24 15 9 2.4
    10 | 300 186 118 74 47 30 19 12 3
    15 | 440 280 176 111 70 44 28 18 4
    | CO2 milligrams/liter

    Ok, maybe someone can shed some more light on this matter...

    This shows the relation with KH and PH and CO2

    Now somewhere this has a relationship with the H30 as that is what is being measured by the PH reading... Maybe the more H30 the less CO2?

    Where does GH fit in? where is the Calcium and Magnesium involved.Is that a natural product that is in the water?

    Explain how H3O and CO2 are related - I mean explain how more CO2 increase KH or decrease PH?
    Ca(HCO3)2 & Mg(HCO3)2. how does CO2 comes out of this (or does it gets added?)

    etc... I will also do research myself and if I find the info I will post it here!