Water chemistry in the aquarium

It is possible to keep healthy and happy fish without knowing anything about the foundations for the intricate chemical processes that takes place inside your aquarium. To be frank, a lot of aquarium fish die every year in the hands of overzealous keepers that goes over board in their efforts to tinker with the water chemistry of their aquariums. Altering one single factor can cause a series of reactions and if you do not know what you’re doing, the result can very well prove fatal for your fish. There are however situations where you will be forced to adjust the water chemistry in your aquarium to make it suitable for certain species. Understanding the weird and wonderful chemical and biochemical processes that goes on inside your aquarium will also provide you with a deeper understanding of the mesmerizing aquatic world and allow you to see the hobby from a new angle.

Chemistry and atomic physics 101

As you probably know already, the atom is the smallest indivisible unit of a chemical element, and a molecule is the smallest indivisible unit of a chemical compound.

An atom can be broken up into even smaller particles: neutrons, protons and electrons. (No, we will not try to break up any neutrons, protons or electrons into smaller particles in this article.) Neutrons and protons form the central nucleus of the atom and are surrounded by a cloud of electrons. Separating neutrons and protons from each other is quite complicated and requires a really large amount of energy, while removing electrons from the electron cloud is an entirely different story. Every time your brush your hair and it becomes electric, you are actually removing electrons from their cloud. If you remove electrons from a chemical compound, it still remains the same compound, only with a different charge.

Protons are said to have a positive charge, while electrons are said to have a negative charge. Since protons have the same charge they will repel each other, but the nuclei of our atoms do not fly apart –why is that? Luckily for us, there are one or several neutrons inside the nuclei of every atom. The neutrons create a force known as “the strong nuclear force” and bind the nucleus together.

The different charge in protons (+) and electrons (-) create a relatively strong attraction between them. Quantum mechanics, which will not be discussed or explained in this article, does however prevent the electrons from being sucked into the nucleus of an atom and arrange the electrons in "shells". The inner shell can optimally contain 2 electrons while all other shells ideally contain 8 electrons. If an atom has more electrons than can be fitter in one shell the excess electrons are placed in another shell further from the nucleus of the atom. A atom with 13 electrons will as an example have 3 shells, 2 in the innermost, 8 in the next and the remaining 3 in the outermost shell making it a very reactive atom. Atoms strive to have the optimal number of electrons in their outermost shell and this is, simplified, why atoms react and bind with other atoms and form molecular bonds. They share the electrons in their outermost shells to achieve the optimal amount. The number of electrons in the outermost shell decides atoms chemical properties. Nobel gases such as neon have 8 electrons in their outermost shell which results in the fact that noble gases can't react or interact with any other atom. They are perfect in their own right. An atom with very few electrons in its outermost shell on the other hand is extremely prune to react with other atoms.

Hydrogen and oxygen are two very reactive atoms. Hydrogen has one electron and therefore need one more electron to reach the optimal number electrons. Remember the innermost shell only have room for two electrons and are complete with just two atoms. Oxygen has 8 electrons, 2 in the inner shell and 6 in the outer, which means it needs two more electrons. An oxygen atom can therefore react with two hydrogen atoms and form water.

There is another way that an atom can get the optimal number of electrons and that is by forming Ionic bonds. This means that one atom steals an electron from another atom and changes the charge of the involved atoms. The atom that takes an electron gets more electrons than protons and becomes negatively charge while the other atom becomes positively charge as it after the reaction has fewer electrons than proton. An atom that has been affected in this way is called an ion. The positively charged atom is referred to as a cation and the negatively charged atom is referred to as an anion. To denote a ion you add + (positive) or – (negatively charged) to the chemical symbol. Change the number of pluses and minuses to denote how strongly charged an atom is.  An atom that lost two electrons would as an example be denoted by adding ++ to the chemical symbol. An atom with many electrons in it outermost shell are more likely to steal an electron and an atom with few electrons are more likely to be the loosing part. This is because atoms with fewer electrons in their outermost shell hold on to those electrons less intensively than atoms with more electrons in their other shell. An atom with only one electron in its outermost shell that losses that electron will after the loss have an other shell with the optimal number of electrons even if they lost a shell.

Positively and negatively charged ions attract each other and form ionic bonds.  Different salts are examples of molecules that are former by ionic bonds. All ionic bonds can be easily solved in water even if the bonds often are very strong.

It's is the number of two different ions H+ and OH- (hydroxide) that decides the pH of the water in an aquarium and understanding the principles behind the presence of these two ions will help you regulate pH in your aquarium more easily and more successfully.

Aquarium pH

Pure neutral water contains the same amount of H+ and OH- (hydroxide).  The amounts are than equal to 10-7 moles per liter. A mule H+ is 6.022x1023 H+ atoms.  Most, "all", of the H+ in water binds with water molecules and form H3O+ and it technically the amount of H3O+ molecules in the water that decides it's pH. The more H3O+ molecules the lower the pH.

Water with pH <7 is referred to as acidic and water with ph >7 is referred to as alkaline. The term alkalinity and acidity is sometimes incorrectly used as synonyms for this terms but the terms alkalinity and acidity in reality refers to the waters ability to resist pH changes upwards (acidity) or downwards  (alkalinity). It important to keep these apart as water can be acidic without having high acidity or alkaline without having high alkalinity. You should strive to have high acidity in your aquarium if you have an acidic aquarium and vise versa as this prevents rapid changes in the pH in the aquarium. Most fish can live in less than ideal pH conditions as long as the pH is still but all fish have hard time coping with rapidly changing pH levels. It is not an understatement to say that a stable pH usually is more important than the right pH when keeping fish. You can give your aquarium high alkalinity by adding limestone in the aquarium or in the filter. High acidity can be achieved by using peat moss.

Alkalinity is usually measured in KH which is old outdated unit and it is better to measure the amount of calcium carbonate  in milliequivalents per liter (meq/L), or in milligrams per liter. You can convert KH to meq/L by multiplying kH by 50.04 or to mg/L by multiplying KH by 17.8

Aquarium Hardness

Aquarists talk about several different types of water hardness. If you just say water hardness you are usually referring to the number of multivalent cations in the water. A multivalent cations is an ion that stolen two or more electrons from some other atom and therefore has become multiply charged. I.e.  a ++ ion. There exist a long row of different multivalent cations but only two of them are common enough to be worth measuring, Ca++ and Mg+. Single charged cations don't only affect water salinity and not water hardness. (At least not directly) Water hardness are most commonly measured using the DH scale but this is just like the KH scale an outdated and obsolete scale. It is better to measure hardness in mg/L. Convert from DH to mg/l by multiplying DH by 17.8

Other hardness measures is carbonate hardness or temporary hardness and permanent hardness.  Carbonate hardness is sometimes is called KH. It has however nothing to do with pH or the alkalinity and should not be confused with the earlier mentioned KH scale. I will however not discuss temporary or permanent hardness in this article.

It is important to remember that although water hardness, pH and alkalinity all are different things they all interact and affect each other.  I will not discuss how here but there are plenty information available on this topic on the internet.

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