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Hardness: A Term with Many Meanings

by Timothy A. Hovanec

The word hardness is part of several water quality phrases which have little to do with one another and seem to be a constant source of confusion for hobbyists. Two of the most common phrases are carbonate hardness and total hardness. Carbonate hardness, from the archaic German term of the same name, is the same as alkalinity or acid-neutralizing capacity (ANC). This was the subject of my February 1995 column and will not be covered here in detail. But, to briefly review, alkalinity or ANC is the amount of buffering capacity in a water. It is mostly the result of bicarbonate and carbonate compounds and can change over a short period of time as the acids produced in an aquarium (via nitrification, for example) consume the ANC. Eventually, the ANC will become zero and the water pH will drop to a relatively low value because the water has lost its buffering capacity. To replenish the ANC, buffers such as sodium bicarbonate (baking soda) can be added to the water.

Therefore, knowing the pH value will not tell you what the alkalinity is and conversely knowing the alkalinity will not tell you the pH. While some may say that you can generalize that a high pH means you have a lot buffering capacity and a low pH means you have none, this can be wrong. I once managed a fish farm run with well water that had a pH of nearly 9 but it had almost no alkalinity. A small addition of acid would cause the pH to drop immediately. It is better not to assume and, instead, measure the water values.

A potential confusion with ANC and total hardness arises from the fact both ANC and total hardness values are usually reported in equivalents of calcium carbonate (CaCO3). In the case of alkalinity, it is the carbonate (CO3-) fraction of the term we are interested in, which differs from total hardness as will be explained below. The units of measurement can also be confusing. Reporting CaCO3 does not mean that all the alkalinity (or total hardness) is in the form of calcium carbonate, what it means is that all the compounds contributing to alkalinity (or total hardness) have been mathematically converted to units of CaCO3 to make it easier to report just one value. If this were not the case, then one would have to report separate numbers for bicarbonate, carbonate, and the other compounds which contribute to alkalinity.

The second term, which is the main theme for this month's column, is total hardness or sometimes called general hardness. It is the measure of the amount of calcium (Ca++) and magnesium (Mg++) in the water. In the past, and outside the aquarium hobby, water hardness measurements were primarily concerned with the capacity of a given water to cause soap to produce suds, but this is no longer in use.

Like alkalinity, total hardness is usually expressed in equivalences to calcium carbonate (CaCO3) such as, "the water hardness was 50 mg/L as CaCO3." But in this case, it is the amount of calcium and magnesium in the water that are standardized to units of CaCO3 by multiplying the Ca++ value (mg/L) by 2.497 and the Mg++ value (mg/L) by 4.118. The best way to determine the hardness of a water is to separately determine the amounts of calcium and magnesium in the water and do the conversion. A more common, easier way to determine water hardness is by titration with sodium EDTA in the presence of a dye such as Calmagite. As the EDTA is added, the Ca++ and Mg++ are complexed and when complete, the solution turns blue. The number of drops or mL of titrant are calculated and multiplied by a conversion factor to produce a hardness value in CaCO3. Soft water is defined as water with a total hardness less than 50 mg/L CaCO3, moderately hard water is around 100 mg/L CaCO3 and hard water is around 150 mg/L CaCO3. (For drinking water, usually values above 150 CaCO3 start people to complain about taste.)

Another measurement term for total hardness still used in the aquarium hobby, but not many other places, is German degrees of hardness (dH(). In actuality, one dH( equals 10 mg of lime (CaO) per liter of water. But with a little math, it can be converted to 7.14 mg Ca++/L. Using the 2.497 number mentioned above, this can be converted to 17.83 mg CaCO3. Therefore, one dH( unit equals 17.83 mg CaCO3. The important point to keep in mind is that there are many ways to report the values and if you have a preference, stay with it. But when making comparisons with other hobbyists or trying a new test kit make sure the system of measurement is the same as the system you have been using.

Total hardness usually does not decrease in the aquarium environment. In fact, it will probably increase over time to high values. This happens because when evaporation occurs only water, not what was in the water, disappears to the atmosphere. This has a concentration effect, thus increasing the total hardness. Unlike the case of alkalinity, there are few biological or chemical processes in a freshwater aquarium which consume total hardness. In a seawater reef aquarium, reef building corals use calcium from the water to build their reef structure, a process called calcification. Therefore, calcium is added to reef tanks on a regular schedule.

The common question facing the hobbyist is: should I adjust the hardness of my water? A case can be made for both adjusting or not adjusting. For the great of majority of hobbyists, no adjustment of hardness is needed. The reasons are: 1) it is unnecessary for most fish; 2) once your aquarium has been adjusted, adding more fish to the tank becomes problematic; and 3) it adds unnecessary expense and complication to the hobby.

For the average hobbyist trying to maintain the more commonly available fish, the tap or well water in most of the United States is fine, in regards to hardness. If in doubt, ask your local fish store manager if they do anything special to the water for their fish. If not, then you should not have to either. Generally, tropical fish stores do not adjust their aquarium water, so the fish that come from them are accustomed to a certain hardness. If you have adjusted your hardness, when you take the fish home you have to spend time acclimating them to your water hardness. If you chose to adjust your water, you will have to invest in ion exchange resins or water treatment systems such reverse osmosis (RO) or deionization (DI), all of which can be costly.

If you are sure you want to adjust your water hardness, then you need to understand the differences between alkalinity and total hardness, and how different treatments are used to reduce or increase either or both of these. For example, ion exchange resins are available to remove total hardness from aquarium water which may be necessary for rearing certain fish. But be careful using these products for fish such as South American tetras. Certain resins use an exchange process which removes calcium and magnesium, but puts sodium into the water. These fish do not tolerate much salt (sodium) in their water. Not all ion exchange resins use a sodium based resin, but be sure to read the label to see if the product is safe with these fish.

Removing calcium and magnesium via ion exchange resins will not affect the alkalinity of the water. The pH should stay the same if you use a sodium based resin. But if you use a different type of resin the pH may be affected, so it is best to test the water before placing it in your aquarium. Also, if you use RO or DI to remove total hardness, alkalinity will also be removed and the pH will change. In this case, you may need to add back certain chemicals to get adequate alkalinity. For instance, adding sodium bicarbonate will increase the alkalinity, but not the total hardness. Adding calcium carbonate will increase both.

A certain amount of total water hardness is needed in aquarium water as it is important to fish health. Granted, there are some fish that live in very soft water but even in most of these cases there is some hardness. Calcium is important because it assists in the osmoregulation of fish. Further, total hardness decreases the toxicity of metals dissolved in the water. This is why medicating with copper, for instance, has to be done very carefully in soft waters. In hard water, the calcium decreases the rate of copper uptake by the fish which means a larger amount of copper is needed to be effective. But in soft waters (little calcium), the copper uptake by fish is much better and, therefore, less copper should be used or the fish will die from copper poisoning.

It is important to remember the difference between total hardness and alkalinity (carbonate hardness) as they affect water chemistry and fish differently. Further, neither is an indicator or measurement of pH. A basic knowledge of these three important water chemistry characteristics should help clear some of the confusion in the hobby today and help you understand some of the basic processes occurring in your aquaria.

(c)1996, Timothy A. Hovanec
Originally published in Aquarium Fish Magazine, May. 1996

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