The Stuff That Water’s Made Of
Part 1: The Molecule, Temperature, Saltwater, and Osmosis
by Lenny Llambi
First published in Fincinnati, the official newsletter of the Greater Cincinnati
Aquarium Society
Aquarticles.com
One crucial component of the aquarium hobby that helps the hobbyist in his/her venture
is knowledge. Aquarium keeping is an interesting hobby in that it combines principles from
physics, chemistry, and biology. The more we understand each of the principles within
these disciplines, the easier our hobby becomes. Of course, biology is the most understood
of the three fields, because lets face it our final goal is to maintain living creatures.
However, much like junior year in high school, we need to understand a basic level of
chemistry to survive (or for our fish to survive as the case may be).
In this series of articles, I hope to put many of the chemical principles behind
aquarium keeping into laymen’s terms. In future articles I will discuss subjects such
as the nitrogen cycle, dissolved oxygen, and pH. The more we understand each of these
factors in water chemistry and why they are good or bad, the more we understand our hobby
as a whole. This issue’s installment is going to start at the beginning: water. Water
is such a basic element of the aquarium hobby that it is often overlooked. However, it is
no coincidence that water is the molecule which makes up the oceans, lakes, and rivers
from which the fish we keep originate. It is because of the way that water interacts with
other chemicals, itself, and our fish that the majority of life on this planet is
sustained within this amazing matrix.
Water: The Molecule
Water is a molecule unlike any other. It is made up of one oxygen molecule and two
hydrogen molecules. Each hydrogen forms a bond connecting it to the oxygen molecule,
forming an “upside-down V” shape. If you look at the upside-down V shape of
water, you will notice that the two hydrogen atoms (which have an overall positive charge)
are positioned across from the oxygen atom (which has an overall negative charge). This
alignment means that one side of the water molecule (the oxygen side) has a slight
negative charge and vice versa. Conversely, each side will then attract molecules or ions
of opposite charge. This is known as the dipolar nature of water.
At this point you’re probably having high school flashbacks, because: “Why in
the world did I need to know all that, and what will I ever use it for!?!?!?!”
Water’s dipolarity is exactly what makes it such a great solvent for the laundry
list of chemicals with which we aquarists worry ourselves: nitrate, ammonia, oxygen, iron,
calcium, etc. All of these molecules and ions have one thing in common: they have a charge
(the charge can even be a partial charge like water’s). Imagine, in your mind’s
eye, a lone Calcium ion that was just dissolved into your reef aquarium by a calcium
reactor. The Ca2++ has a positive charge, so when a group of water molecules encircle the
calcium ion with their negative ends pointing in…Voila! The calcium ion has been
dissolved.
This brings up a vital factor to the well being of your fish. Due to the way that it
must organize itself in order to dissolve a chemical, water has a finite amount of space
available for dissolving. Often times nitrates are brushed aside as being not dangerous to
freshwater fish. However, a tank which has been ignored and allowed to build up nitrates
will have significantly “less space” to dissolve critical molecules like oxygen
or carbon dioxide.
Temperature
Water’s ability to dissolve is also determined by its temperature. Aquarists are
interested in dissolving chemicals in every state of matter, whether it is a solid,
liquid, or gas. As water temperature increases, solids and liquids tend to dissolve more
readily. However, as temperature decreases, gases tend to dissolve more readily, which
makes a real balancing act out of maintaining water temperature. In every aquatic
environment gases, liquids and solids are essential to fish health. If you think about it,
there are really very few fish that require temperatures above eighty degrees. Now you
understand why keeping dissolved wastes down is so important when keeping discus at the
warm temperatures they enjoy (well, actually that’s only one of many reasons). When
phosphates build up due to a poor water change regimen and overfeeding, the amount of
phosphate dissolved in the aquarium directly decreases the amount of “free room”
water has to dissolve oxygen for the discus to breathe. Oxygen depletion has many
deleterious effects on all fish, but the discus, which has evolved in oxygen-rich streams,
will be doomed in water unable to dissolve much oxygen.
Saltwater
Those of us who keep marine aquariums are especially affected by the dipolar nature of
water. One of the questions I was fielded the most as a pet store, fish clerk was:
“Is saltwater really as hard as they say it is?” Now I could have delivered this
article in a brilliant oratory transcription, but a) the customer was probably just
curious b) “yes” is a whole lot quicker. In fact, the reason that
“saltwater is as hard as they say it is”, is due to water’s dipolarity.
Saltwater, of course, must first dissolve a series of salts to fulfill the proper salinity
before it can begin to dissolve other essential molecules or ions. While most of the
people who approached me about starting a saltwater would consider the proper maintenance
behind a marine aquarium overbearing, the knowledgeable hobbyist has a number simple of
methods at his/her disposal to keep the tank free of excess waste molecules. Protein
skimmers, macro-algae cultivation, and deep sandbeds are just three methods aquarists use
to remove excess, waste byproducts.
I personally have a 29-gallon mini-reef aquarium, which has been a constant bear;
because of saltwater’s reduced ability to dissolve essential molecules. A 65W power
compact light fixture and a 20W normal output light fixture light the tank. This amount of
light raises the temperature into the mid-eighties, at which point all the corals begin to
bleach from insufficient dissolved oxygen. I use a fan to keep the temperature down, but
this causes a great deal of evaporation. The confines of my small, one-room apartment
prohibit an automatic top-off system, thus requiring me to manually top-off the aquarium
with kalkwasser throughout the day. This is merely one obstacle in the three-ring circus
balancing act that is my mini-reef aquarium.
Osmosis
All of these ions and molecules that we are dissolving in water don’t just “sit
there”. They are constantly moving so that they are equally dispersed throughout a
body of water. Imagine a ten-gallon aquarium, filled with distilled water, divided into
two compartments with a piece of steel. If only one side was adjusted to a salinity of 33,
as soon as the piece of steel is removed, the salt ions would slowly distribute themselves
so that the entire aquarium registered a salinity of 16.5 (exactly half of 33). This
phenomenon is called diffusion.
All of the salt ions, bouncing off of the steel divider, were exerting a certain amount
of pressure on the piece of steel, specifically called: osmotic pressure. The higher the
salt concentration, the greater the osmotic pressure. Steel is an incredibly impermeable
material, but our fish’s cellular membranes are not. In order to prevent a virtual
implosion, our fish’s cellular membranes are actually permeable to small ions and
water. This means that if the water outside of a fish’s cells has more dissolved ions
than the water inside of its cells (i.e. osmotic pressure is greater outside of the
fish’s cell); the fish’s cell membrane allows small ions to diffuse into the
cell and water to exit the cell until the osmotic pressure is equalized. Every living
creature that makes water its home has to be able to deal with osmotic pressure in this
manner. Although we take it for granted, this is probably the cell’s membrane most
important function.
After thousands, if not hundreds of thousands, if not a million years, of adaptation,
all aquatic creatures are used to a certain amount of natural fluctuation in osmotic
pressure. This is another illustration of why saltwater is deemed: “harder than
freshwater”. Since the vast volume of the ocean provides for pretty consistent water
parameters, corals, sea stars, butterfly fish, etc. are very intolerant of changes in
salinity, pH, etc. However, some freshwater fish actually require relatively drastic
changes in water parameters to breed (some goes as far a requiring no water for a short
period of time, ala many killifishes). Nonetheless these changes happen within a certain
range, and due to years of evolution, anything outside of this range is often lethal.
Hopefully all of this painful chemistry sheds a little more light on the
“fish-keeping experience”. We now know that water’s dipolarity makes water
one of the best solvents on earth, however, with a limited capacity. We’ve also
learned that the more molecules that water has to dissolve, the less space water has to
dissolve essential ions and molecules for our fish. The higher the water temperature, the
more readily solids and liquids will dissolve, while conversely limiting how much gas can
be dissolved. Finally, as the molecules and ions that water dissolves move about, they
exert a certain amount of pressure on our fish called osmotic pressure. Stay tuned for the
next episode, when I will cover everything you ever waned to know about conductivity and
water hardness. These two water parameters measure how many molecules and ions are
dissolved in water, and directly reflect the amount of osmotic pressure exerted upon our
fish. In the meantime let’s keep learning about and caring for our fish.
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