The Stuff That Water’s Made Of
Part 2: Conductivity and General Hardness
by Lenny Llambi
First published in Fincinnati, the official newsletter of the Greater Cincinnati
Aquarium Society
Aquarticles.com
In the previous article we discussed water’s ability to dissolve ions and
molecules (solutes) is determined by the fact that the water molecule has two polar,
opposite charges (dipolarity). We also examined how our fish are able to pump water and
other small molecules across their cell membranes, in order to match the concentration of
solutes within their cells to the concentration of solutes outside of their cells. In this
edition we will examine two water parameters which measure the amount of solute (to
varying degrees) in your aquarium water: Conductivity and General Hardness.
Conductivity
Conductivity may not be a water parameter that we worry about all day long, but not only
does it need to be explained to understand all other water parameters; I think it needs to
be understood better to become a better aquarist. Conductivity is a substance’s
ability to “carry” an electrical current. We’ve all done the grade-school
experiment with the potato, light bulb, and battery, but did you know that you could
substitute the potato for a glass of tap water? The more ions and molecules that are
dissolved in water (remember a chemical must have a charge in order to be dissolved by
water), the more electrical current that water is able to conduce. As a matter of fact,
conductivity is synonymous with the term Total Dissolved Solids (TDS), which is measured
in parts per million (ppm). In order to convert TDS to conductivity, simply multiply TDS
by 0.64. I hope everyone reading this article now understands why I am covering this water
parameter before I go into other, more common water parameters. I guess you could look at
conductivity, or TDS, as the mother of all water parameters, because it essentially
measures all dissolved ions and molecules.
Conductivity is measured in microSiemens ( S), and is directly proportional to the
amount of osmotic pressure exerted on our fish’s cellular membranes. Distilled water
has a conductivity of 0 S, whereas seawater has a conductivity of about 5000 S. Every
chemical, additive, piece of food, medication, or conditioner you put into your tank
increases the conductivity in your aquarium, once the water dissolves it. This is why it
is essential that any fish that you are adding to your aquarium be acclimated to your
aquarium water before it is released. Your fish needs as much time as possible to slowly
pump the proper amount of water in or out, so that the osmotic pressure is equalized
across its cell membrane.
What Goes In Must Equal What Comes Out
Looking back at my tenure as a fish clerk in several different stores, I realize now how
essential it is to be aware of your water’s conductivity. When I would place a fish
order at one of those stores, the supplier would bag several well-fed (i.e.
waste-producing) fish in a little bag with water from a tank with, lets say, a
conductivity of 500 S. He would always use a very thick stress coat and drop in a tablet
of some sort of methyl-blue prophylactic, probably increasing the bag to a conductivity of
650 S. Eight hours later, when the shipment arrives, this bag of waste-producing fish has
a conductivity of 1000 S. Since our fish supplier was local, our tap water had similar
conductivity readings; so when I received this order of fish I had to slowly reduce the
conductivity by fifty percent.
That store had an automatic top-off system so daily water changes maintained a fairly
constant conductivity. However other stores are not so generous with water changes. This
becomes a problem in overcrowded and overfed tanks, especially when the top-off water is
tap water of high conductivity. I have read about people buying freshwater fish that were
being kept in 5000 S water at the store. Although this is probably an extreme, when you
watch all of the stress coats, aquarium salts, and medications that are indiscriminately
poured into some store tanks, its not such an extreme number. This is one reason why,
especially in smaller tanks, you should not add water from the bag you received from your
local fish store. Now if you purchase a fish from a fellow aquarium society member,
you’re probably safe assuming that their water-change regimen is keeping their
conductivity at a comparable level. Even then, if you ever acquire any of my fish, you may
want to get a feeling for how busy my life has been (that is also directly proportional to
my water’s conductivity).
Conductive Spawning
As I mentioned before, many fish are capable of withstanding a broad range of conductivity
levels. These fish are also more resistant to changes in conductivity. Many of these fish
actually require changes in conductivity in order to come into breeding condition. Two
fish species, whose breeding behavior is very elusive, exemplify two extremes. Monodactylus
sebae, which can live in fresh as well as marine water, was recently reported only
breeding in seawater where its microscopic, larval progeny can enter the planktonic drifts
in the ocean. Once a larva reaches the free-swimming, fry stage, it is able to swim
further into fresher waters, as it grows.
Botia macracanthus (the Clown Loach) has been reported spawning, accidentally,
after its aquarium is “neglected” (i.e. not fed often, not topped off, and not
cleaned), then pumping the aquarium full of distilled water after a large water change. By
not performing water changes, nitrates and phosphates build up and increase the
conductivity. Moreover, ceasing to top-off any evaporated water decreases the amount of
water dissolving the solutes in the water, once again increasing conductivity. When a
water change is performed, and the aquarium is topped-off with distilled water, the
conductivity drops drastically, and the clown loach begins its elusive spawning behavior.
This method replicates the dynamic between the dry season and the rain season (when the
clown loach breeds).
General Hardness
General Hardness (GH) is a misleading term, because it is actually derived from the
German: Gesamt Haerte. It is often confused with a term that we will discuss in the next
edition of this article: Carbonate Hardness (KH). Since KH is actually alkalinity, not
hardness, the term general hardness should probably be abandoned for the simpler term:
Hardness. Water hardness measures the amount of ions which have two extra protons
(divalent cations) dissolved in our aquarium water. The most common, divalent cations
(almost to the exclusion of all others) that make up a Hardness reading are Calcium and
Magnesium. These two ions are essential for bone and scale formation, blood clotting, the
importation of other ions, electrical current transfers in nerves and muscles, and
numerous other metabolic processes.
Just as confusing its name are the terms we use to measure general hardness. The two
most common units used are ppm (more common in the U.S.A.) and dH (German degrees of
hardness). In order to convert these two parameters, simply use the following equations:
dH x 17.9 = ppm ppm x 0.056 = dH
Many popular fish species such as: discus, dwarf cichlids, and killifish come from
soft, and acidic environments, and often require these same parameters to breed. Although
the GH readings in these environments are well below those found say in the African rift
lakes, a certain amount of hardness needs to be maintained, because of calcium’s and
magnesium’s importance to fish development. Fish take in calcium and magnesium mainly
through their gills, so these two ions need to be present in water for our fish to develop
properly. Actually, if you ever invest in a Reverse Osmosis/De-Ionization unit (RO/DI), it
is best (and probably cheaper) that you purchase a unit that is manufactured for use by
aquarists. These units do not produce pure water (0 S) Instead they produce “product
water” (20-30 S), which simply means that the water has a minimal amount of ions in
the water, typically: magnesium and calcium.
Even though most of us don’t concern ourselves with conductivity and general
hardness on a day-in day-out basis, our fish certainly do. Conductivity is probably the
number one reason fish get stressed when being transferred from one environ to another.
The more conductive water is the more osmotic pressure that is exerted on our fish.
General hardness may not immediately kill a fish when the parameter falls below a
particular level, but it will hamper fish development and many biological processes. This
isn’t the first time I’ve said it, and it won’t be the last: we must always
remain conscientious of where our fish originate, because this determines what water
parameters are acceptable to our aqueous pets. A discus may readily prosper in our
unnatural (for discus), Cincinnati tap water (liquid-rock as many of us refer to it).
However, you will be hard-pressed to nurture a young Julidochromis marlieri to a
healthy adulthood in water of low general hardness. Next issue we’ll venture into
some familiar water by exploring pH and the “other hardness”: carbonate
hardness, or alkalinity. In the meantime let’s keep learning about and caring for our
fish.
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