The Effect of pH (and not only) on Fish

The Effect of pH (and not only) on Fish

PDF | Print | E-mail

pH (Hydrogen Potential) is chemically defined as the negative logarithm of the concentration of Hydrogen ions in a solution. This, although accurate, means very little to the average fish keeper. We need to simplify things a bit and then examine why pH and other fluctuations of the water chemistry may easily kill living organisms.

Water consists of two molecules of Hydrogen (H) combined to one molecule of Oxygen (O) as we all know. What most fish keepers don't know is that a very small portion of water molecules dissociates and gives equal numbers of hydroxyl groups (OH-) and hydrogen ions (H+). It is evident that only equal numbers of OH- and H+ can exist in pure water. Actually in distilled water, only 10-7 molecules dissociate (one molecule of water every 10.000.000 molecules) providing 10-7 OH- and 10-7 H+. This in turn means that water is only slightly soluble in water!!

The second critical factor to remember is that the multiple of OH- x H+ is constant for water and aqueous solutions. Accordingly, the more H+ ions in such a solution the less OH- there is in it and vice versa.

The negative logarithm of 10-7 is 7, therefore distilled water has a pH of 7 which is called neutral, meaning that equal numbers of H+ and OH- are present or, in other words there is no surplus of either ions or negative groups.

If acid is added in the water ("acid" is defined as a compound that releases H+ when dissolved in water) then the H+ molecules will accumulate in the water. Thus the concentration of H+ will become (as we keep on pouring acid) 10-6 (one H+ ion every one million molecules), then 10-5 (one ion every 100.000 molecules) then 10-4 (one ion every 10.000 molecules) or even more. Of course the pH will gradually change from 7 to 6, then to 5 and 4 or even lower. Any solution with a pH less than 7.0 is called acidic. The lower the value the stronger the acidic character. Concentrated acids have a pH of nearly 0. At that pH only H+ exist.

Exactly the opposite will happen if a base is added (a "base" is a compound that releases / produces OH-). Since the total multiple must remain the same, as indicated, the more OH- in the water, the less the H+ will be. Thus the concentration of H+ will go from neutral (one every 10.000.000 molecules) to less and less H+ (one every molecules or even less). One every 1 billion molecules means 1X10-9 which means a pH of 9. Any solution with a pH over 7.0 is called basic or alkaline. Strong bases (NaOH, KOH etc) produce solutions with a pH value near 14. At that pH there is no H+.

Why is this important? For a number of reasons. Most importantly, a fluctuation of 1 point in the pH scale means a tenfold increase or decrease in the H+ ions present in the solution. This has implications on the fish.

Take the example of a hobbyist making a water change using tap water with a pH of 6.5. Additives (chemicals) are subsequently added to the water to bring the pH back up to 8.5 for the African cichlids in the tank. In such a case it is well possible to find some of the fish dead next day. Why?

The fish were accustomed to a certain H+ concentration. Then, suddenly, they are facing new water coming in their tank, which only contains one hundredth of the normal H+. This is a shock by itself, a very serious shock. While in stress, the fish try to adapt to the new situation when suddenly something is added in the water which creates a new solution with 100 times more H+. No organism can adapt to this sort of fluctuations!!

To understand why we need to look at what happens in the cells of the fish or plant. Very simply, the cell is like a membrane which is permeable. In short, there is a narrow limit of differences in concentration that the membrane can handle. If the outer concentration suddenly raises 100 times then the cell has to react. It does so in two ways. Either by releasing water (so the concentration inside the membrane increases, too) or by absorbing as much material as it can handle in order to level the (internal and external) concentrations.

However, the cell is a living unit. It is not a lifeless membrane which can stay intact. There is a limit on how much water it can expel or how many H+ ions it can cope with in the inside (cytoplasm). With fluctuations of this magnitude, most cells simply can't cope. This is even more pronounced if the fluctuation is instant. Cells have a remarkable adaptability to their surrounding and can cope with this kind of fluctuations if they are gradual.

A great way to reduce this sort of fluctuations is the use of buffers which have the ability to "absorb" the influence of an acid or a base and keep the pH relatively stable. This is not valid for pH only. It is also true for the GH, KH, conductivity, alkalinity etc.

These two entities (GH & KH) show how much calcium, magnesium or carbonates are dissolved in the water. Again, an instant raising of the GH from 10 to 20 will cause too much stress to your fish. The living cell has a certain osmotic pressure in the interior (proportional to the concentration of particles in the cytoplasm) and has reached a dynamic equilibrium with the surrounding osmotic pressure.

The aim of every cell is to minimize the differences between its interior and the exterior pressures or keep a specific difference. Obviously, the sudden addition of salt in the environment causes the cell to counteract immediately in order to survive, which means it has to absorb salts at dangerous or even fatal levels. This is due to the fact that the cell needs to ensure its survival first and then deal with the extra salts it has accumulated. Again, a gradual increase will allow the fishes to adapt to values that would kill them if applied instantly. Bear in mind that not all species have the same kind of cells, i.e. cells that have the same tolerance or can survive under the same conditions. It is obvious that when cells are exposed to conditions outside their tolerance range they die. For example an African cichlid from Lake Malawi may survive for months or years at a pH of 9.2 with a GH=30 (though not the optimal conditions) whereas a discus will die very shortly!

There are two rules which, if followed, will ensure fish won't be harmed by fluctuating water chemistry:

Rule 1: Know your fish and the range of conditions they should live in. Every single species carries its own genetic code; this is information on how its cells should be built. This genetic code is equally valid for all members of a species, whether wild caught or tank bred; it has been naturally selected as the "best" millions of years ago and remains unchanged. Now cells (and subsequently tissues, organs and organisms) may adapt to a wide range of external conditions but not without a cost. Adapting means starting, stopping or modifying something, perhaps by "irregularly" activating or inhibiting a biochemical pathway. Experienced fish keepers usually keep fish from the same habitat in their own tank and they try to mimic nature as closely as possible.

Rule 2: It is highly recommended to dissolve the total quantity of salts or other additives (e.g. pH buffers) in a couple of liters of water and then add the solution little by little as the new water comes in your tank. This will greatly minimize fluctuations. Avoid any instant corrections or alterations of the conditions (temperature, lighting etc) and especially of the water parameters.

Update, March 2006

In a "normal" tank, pH is directly linked to carbonate hardness (KH) and the quantity of carbon dioxide dissolved in it. A formula allowing you to regulate the KH and CO2 content of your water in order to get the desired pH can be found  here. This sounds very simple and easy and it is, under normal circumstances. However, there are some misconceptions related to these parameters; being aware of these will help hobbyists to avoid mistakes which may harm the health of fish and plants.

Most people believe in the concept of "soft water = low pH, low General Hardness (GH) and low KH". Some biotopes call for such a combination (most notably the Amazon one) so there is a great number of hobbyists trying to recreate it. Let's look at some issues related to this concept.


To start with, the terms "soft" and "hard" refer to General Hardness (also called permanent hardness) and not to KH. This kind of hardness is, in the main, the result of the presence of Calcium, Magnesium and Iron cations in the water; the values of these are expressed as carbonate equivalents. "Expressed as" means that we use the weight of their carbonate salts in order to make our measurements and identify the GH levels of our water. It does not mean that what we really have in our water is Calcium or Magnesium carbonate. We may or may not have it. If we have an equivalent amount of Calcium chloride for example (CaCl2) then we will have the same GH even though we don't have a single carbonate radical (CO32-) in our water. This is extremely important to note. GH has to do with calcium and magnesium cations and not carbonate radicals, so GH is completely independent of the pH of the water and totally different to KH. Please note this carefully: GH has nothing to do with the pH. You may have an extremely high GH and still have an acidic pH - in fact, you can have any combination of pH  - GH values. If you have a mixture of calcium chloride with hydrochloric acid, you have a pH close to 1 and a GH as high as you want it to be (even more than 200). In this case you have a very hard and very acidic water. You can also have a solution of potassium hydroxide (KOH) which will result in the softest and most alkaline water you will ever see (GH=0, pH close to 14).


Carbonate hardness  depends on how many carbonate radicals (CO32-) exist in the water.  This is directly related to the pH of the water, since carbonate radicals make a buffer with the CO2 from the atmosphere (or the one injected in the tank) so we have a CO32- > HCO3- > CO2  system. The three entities in this system will interact; if you raise the levels of one there will be corresponding changes to the values of the others as a consequence of this rise. For example, if you add some CO2, some of it will be changed to HCO3- and a small part of that will be changed to CO32- to keep the ratios of these three entities balanced. Thus, the overall conditions in the tank will remain stable (up to a point). In simple words, this carbonate buffering system will keep your pH value stable - within some limits of course. Even if it is not able to keep it stable, still it will not allow it to increase or decrease rapidly. It will act as a pillow, making those changes very gradual. This system, depending on the amount of carbonates in the water, will "absorb" the changes; this is called "buffering capacity". In short, the higher the KH, the more CO2 you can add to your water without any serious changes in the pH of your water. The lower the KH you have, the more prone your system is to sudden and very significant pH changes (pH swings or even pH crash). This is very important in systems which employ a supply of carbon dioxide injection.


Thus, people aiming at a very low KH with a carbon dioxide injection should be very careful because in reality they are always at a razor's edge. If something goes wrong, there is nothing to stop their pH from plummeting or sky rocketing. There are many factors which can have such an effect. The wrong type of stones and rocks, a sandy substrate full of calcium compounds, a dead decaying fish, dying plants and many more.


It should be noted that a swing of the pH around the normal value of 7 (i.e. a pH value swinging from 6.5 to 7.5 in a planted tank with carbon dioxide injection) has one more (potentially lethal) consequence. Ammonia changes to ammonium and back. However, the toxicity potential of the two is not the same. Ammonia is much more toxic than ammonium. Consequently, the environmental conditions in the tank range from safe to non-safe during the day.


To avoid this, we would recommend a KH of 4 as the minimum you can be safe with. Some claim KH=3.3 to be the minimum safe level but we regard 4 as the minimum - especially for a planted tank with heavy carbon dioxide injection. It is true that lower KH levels can be safe for fish but this statement is not unconditional. We have kept fish for long periods of time in water with KH<1 for various reasons (usually related to spawning). To note but one example, George has kept discus in water with a GH=0.5,  KH=0, pH=6.0 for months but had to change their water every day, there was no carbon dioxide injection and he was taking daily pH readings.


It should be noted that KH is not linked to GH in any way. Thus, in the previous example the addition of calcium chloride / hydrochloric acid will result in the following water parameters: GH> 200, KH=0, pH=1 and no buffering capacity at all. In contrast, a strong solution of Na2CO3 (sodium carbonate) or NaHCO3 (sodium bicarbonate) will result in the following parameters: pH = 8.4+, GH=0 and KH> 100 with a huge buffering capacity; just note that the pH will remain over 8.4 even if you have dissolved 12 ppm CO2 in it. Practically, you will have to dissolve more than 300 ppm CO2 in this kind of water just to make the pH neutral (7.0). Of course, nothing can live in this kind of water, it is just an example of extremely soft, very alkaline water with a lot of CO2 in it.


The formula which links Carbon dioxide, carbonate hardness and pH is a very useful one and allows you to predict what the final conditions will be. It will directly show you how much carbon dioxide is dissolved in the water so you can increase or decrease the amount injected to get optimum CO2 levels.


However, this formula only works if your buffering system relies solely (or, at least, mainly) on carbonates. If it doesn't, the formula won't work at all, so you can't use it even to get an approximate estimation. This is definitely the case when pH-up or pH-down additives are added in your tank. This is the main reason we do not recommend the use of them. Most of these additives use phosphates to buffer the water at the desired level. As a result there are two competing buffering systems in the tank; the aquarist can never be sure which one will take over at which stage. This in itself undermines the buffering capacity of your water since it essentially relies on two systems. The same holds true when tannins or humic acids are added, or during extensive peat filtration. Usually, the effect of those agents is minimal and the carbonate system is still the major player in your system but when "too much" of the softener is added, then the situation may change.


In short, hobbyists trying to recreate an Amazonian environment should bear in mind that this is a natural system which can't be replicated in a tank. It relies on the low KH, but also relies on tens of other chemical agents which are dissolved in it thus keeping the pH slightly acidic without a CO2 injection. This is another important fact: when a fish should be kept in slightly acidic water this should be ideally done without CO2 injection.


We hope the above will clarify matters and assist hobbyists in achieving their required water parameters while maintaining a safe environment for their fish and plants.