NEWS104
37
Hardwater is indispensable for the successfulmaintenanceof corals,as they require calcium carbonate to construct their skeletons.
carbondioxide (and/or carbonicacid),and
this equilibrium is, inter alia, temperature-
dependent. If the temperature rises then
the solubility of carbon dioxide in the
water decreases; the carbon dioxide
escapes and the bicarbonate forms a
white, rock-hard, virtually water-insoluble
precipitate (lime scale or “fur”). Everyone
knows this phenomenon from the home.
The carbonate hardness is of major
importance in running an aquarium, as
carbon dioxide is constantly produced in
the aquarium through the respiration of
the fishes and plants, as well as through
the activities of bacteria and fungi, but
carbondioxide is also constantly usedup,
as plants require it for the formation of
sugars via photosynthesis. The result is a
constant shift in thepH,as carbondioxide
dissolves in water to produce carbonic
acid. Delicate fishes are harmed by even
relatively small shifts in pH. A stable pH is
one of the most important prerequisites
for the successful maintenance and
breeding fishes.
If thecarbonatehardness isveryhigh,then
the bicarbonate will "capture" practically
all thecarbondioxide;the result isahigher
pH and hungry plants. Some plants can
extract carbon dioxide from bicarbonate.
Insoluble“chalk”thenprecipitates out and
isdepositedasawhiteencrustationon the
leaves.This process is known as "biogenic
decalcification”. Themajority of the plants
grown in the aquarium, however, cannot
do this and quite simply starve in water
with a high carbonate hardness – leaving
aside the fact that white chalky deposits
on the plants do not accord with the
aesthetic sensibilities of the majority of
aquarists and the light requirement of
these chalk-coated plants is greatly
increased.But if the carbonatehardness is
very low, then there is a danger that too
much free carbonic acid will be formed
and the pH will plummet. Such an
occurrence can be deadly for fishes. And
the reverse-case scenario can occur in
heavily planted aquaria with too low a
carbonate
hardness:
namely
if
photosynthesisbegins (as it should)when
the light is switched on in the morning,
then in some situations all the free carbon
dioxide may be used up, with the result
that thepH soars.
Non-carbonatehardness
Some of the alkaline earth metal cations
that cause hardness do not have
bicarbonate as their counterpart anions,
but instead have sulphate (eg gypsum),
chloride,ornitrate.Because thispartof the
total hardness cannot be altered by
temperature, it is termed "permanent
hardness ", as opposed to the "temporary
hardness”
caused
by
carbonate.
Experience has shown that the non-
carbonate hardness has only a relatively
slight influence on life in the aquarium.
However a degree of effort is required in
order
to
identify
the
individual
components of the non-carbonate
hardness, so such analyses are rarely
performed in thehobby.
How is the hardness measured and how
can soft or hardwaterbecreated?
You can learn about this in the next issue
of theNews.Don’tmiss it!
