The Stuff That Water’s Made Of – pH & Alkalinity

After our previous two perhaps somewhat arcane installments, we can finally begin using some more broadly understood terminology in our discussion of water chemistry. We will explore two water parameters for which we have all owned a test kit: pH & Alkalinity. Whether we are keeping African cichlids, a planted aquarium, or a reef aquarium, these two water parameters can spell the difference between a successful aquarium and a disaster-in-a-box (a glass box that is). pH is a measurement of whether your aquarium has excess protons or electrons; and therefore, determines how molecules react with each other and the creatures we keep. Alkalinity is a measurement of how well the water in our aquariums can “buffer” a basic pH, or in other words, how well our water can maintain a basic pH.

pH & Alkalinity


Ask any aquarist what you are measuring when you take a pH reading, and I’m sure well over 90% of responses would go something like: “You’re measuring how acidic or basic the water is.” Although that answer would be absolutely true, there is more to pH than meets the eye. The terms acidic and basic essentially refer to the number of excess protons and electrons that are present in water. Protons and electrons (generically called sub-atomic particles) are two important building blocks for all ions and molecules. Protons produce a positive charge, which naturally attracts the negative charge of electrons, and vice versa. Any atom, ion, or molecule is constantly trying to achieve equal amounts of electrons and protons, because when this state is achieved, the chemical is considered stable. Chemical reactions are the mechanism by which chemicals equalize the balance between how many electrons and protons a particular chemical contains. For instance, table salt is made up of one sodium ion, which has one more proton than electrons (hence the overall positive charge), and one chlorine ion, which has one more electron than protons (hence the overall negative charge). When these two ions form a bond, the sodium ion gains an electron, the chlorine ion’s extra electron has an extra proton to counteract, and the entire molecule now has equal amounts of electrons and protons.

Unfortunately, a pH reading is not as straight forward as: your water has x number of protons and y number of electrons. So I’d like to take this opportunity to apologize to you, in advance, for the massive chemistry headache I’m about to give you. pH does not measure any and every proton or electron, actually it really doesn’t even measure electrons at all. pH actually measures the number of protons that are present in water in the form of the hydrogen ion (H+). The letters p and h stand for the potential of the hydrogen ion. So when your handy-dandy pH test kit measures the pH between 1 and 6.9999; that particular body of water is considered acidic, and it contains an excess of protons. It also means that your water is just rearing to react with negatively charged ions. So, what does it mean when our same handy-dandy pH test kit tells us that our water has a pH between 7.0001 and 14? Well a reading of 7.0000 (neutral) obviously implies that there is no excess of protons, but does a higher reading mean that there is a negative number of protons. Actually, sort of.

A base is defined as any chemical that can accept a proton. In other words, it is a chemical with a negative charge (i.e. an extra electron) that is able to react with any extra protons. So when our handy-dandy pH test kit gives us a reading above 7.0000; we know that our water has an excess of electrons, in the form of the bases dissolved in water. You can think of a high pH reading as a proton debt. If you have a $2,000 MasterCard bill; you would need to pay the $2,000 before you could start putting money into your savings account. Once you pay your credit card off, you then have an excess of money (until the next fish auction rolls around). Likewise, a basic pH needs a certain amount of acid added to it before the water starts accumulating excess protons.

In the introduction, I noted that the pH of water determines how chemicals react with each other and our animals. For instance, if the pH is acidic, meaning there are extra hydrogen ions (i.e. extra protons) in the water, the chemicals in water will tend to react with chemicals with excess electrons more vigorously. The most evident example of this is ammonia toxicity. There are actually two forms of ammonia with which we are concerned: NH3 (ammonia) and NH4+ (ammonium). Ammonium is not very toxic to fish, whereas, ammonia is extremely toxic (I’ll explain this in more depth in the next edition). As you can see, the difference between these two molecules is: Ammonium has an extra proton (H+), which is provided to the ammonium molecule in acidic water. As soon as pH rises above 7.0, the extra proton in ammonium reacts with the extra electrons of the bases in the water, leaving the lethal ammonia molecule. This reaction can be summarized with the equations:
NH4+ + NaOH NH3 + H+ + Na+ + OH- NH3 + H2O + Na+
NH3 + CH3CO2H NH3 + CH3CO2- + H+ NH4+ + CH3CO2-

I cannot stress enough, in each one of these articles, that any creature which we maintain in our aquariums has evolved to deal with water parameters in a certain way. For instance, fish blood has a pH of approximately 7.4. This blood, especially in the gill region, is separated from the water in which our fish live, by one or two cell layers. A dwarf cichlid from a very acidic Amazon tributary, has evolved so that the two cell layers between its blood and the river water are able to “buffer the water pH” before it affects blood pH. If we were to dump this fish into an aquarium with a very high pH, the fish into an aquarium with a very high pH, the fish may not have a mechanism to deal with external pH in the opposite manner. Of course many of our fish either live in waters of mild pH, or waters with mild pH fluctuations. However, with the ever-rising popularity of African, rift-lake species, killifish species, and marine species there are also many fish that live in waters of extreme and consistent pH. These fish are probably the most susceptible to either wildly fluctuating pH, or simply improper pH.

Alkalinity is a seldom-understood water parameter. It is also known as carbonate hardness (KH), and thus allows for confusion with the term General Hardness. Make no mistake GH and KH are two separate measurements. As we discussed in the second article, GH measures how many cations are in the water. Alkalinity, on the other hand, measures how many anions are in the water. An anion is any ion, which has an overall, negative charge. These anions react with any acids (H+) introduced into the water, thus neutralizing them and maintaining a high pH. This is where the definition of a base as a proton acceptor comes into play. Actually, many people consider alkalinity as the total amount of base present in water. In aquariums the most encountered constituents of alkalinity are: bicarbonate and carbonate. We can then define alkalinity as water’s ability to absorb acids without affecting pH.


Alkalinity has three different sets of units, which are the most frequently encountered in the aquarium trade. In the United States Parts Per Million (ppm) is most prevalent; whereas, in Germany, Degrees of Carbonate Hardness (dKH) is the unit most often used. Finally, a more scientific unit, is: Milliequivalents Per Liter (Meq/L). Even though these three measurements have different origins, I have seen all of them used in aquarium literature, without any sort of rhyme or reason. Here is an easy way of converting these units.

I hate to sound like your mother (sorry GCAS moms) but: I cannot stress enough, in each one of these articles, that any creature which we maintain in our aquariums has evolved to deal with water parameters in a certain way. Alkalinity actually helps us respect this inevitable truth in many circumstances. For instance, we are all well aware of the fact that African, rift-lake cichlids come from waters with a pH of around 8.0. Fish waste is acidic by nature, and will therefore slowly lower the pH in an aquarium. However, if there is an abundance of anions in the water, these chemicals, which contribute toward alkalinity, will react with the acids and neutralize them before pH is affected. Instead of constantly testing for pH in a rift-lake aquarium, adding a high-quality buffer will act as an insurance policy against lethal pH drops.

Well you made it! This was probably the most gruesome of all the installments. Don’t worry if you have to read it a few times, because I had to re-write this third part about a dozen times. At any rate, I hope this gives everyone a better understanding of how pH affects the way chemicals function in our aquariums, and how alkalinity can help maintain a certain pH. Next issue we’ll discuss a subject that all aquarists must inevitably become familiar with: the nitrogen cycle. Of course, we’ll go a little more in depth than most explanations go, but I’m sure that you’ll get some new insights on this natural process. In the meantime let’s keep learning about and caring for our fish.

by Lenny Llambi
First published in Fincinnati, the official newsletter of the Greater Cincinnati Aquarium Society

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