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
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.