Myths of pH Shock

Disclaimer: This article is related to pH and osmotic shock in freshwater aquariums. Concepts covered in this article have not been researched by the author for application in saltwater or reef aquarums.

After a several days spent researching the effects of pH on fish I have more questions than answers and I am absolutely surprised at the lack of available viable information. Sure, there are thousands of web sites and forums posting the same old information about “pH shock”… about never allowing more than a .3 change in pH units within a 24 hour period, etc… but I have found absolutely no scientific data to back up this assertion. What I did find is quite the opposite. I found a scientific study that identified fish can survive a 1.7 shift in pH units without difficulty, I found a study published for the commercial fish industry that identifies a shift under 2 pH units should be acceptable, I found a study where fish (rainbow trout) moved from a pH of 7.2 to a pH of 8.5 suffered no ill effects, and I am finding a different train of thought, with a bases in science, that “pH shock” is a myth. There is no such thing. The real issue appears to be “Osmotic Shock”. Secondly, as the pH changes, the toxicity of other substances in the water changes. It is these two issues that are the actual cause of mortalities usually attributed to “pH Shock”.

Osmotic Shock:

Fish maintain their body fluid levels and release toxins via a process known as osmosis, also referred to, when used in a discussion of this process in fish, as osmoregulation. The function of this process is dependent upon the amount of Total Dissolved Solids (TDS) in the water. For a better understanding of this process, and the relationship it has with TDS, review the following:

The catch, as it relates to this discussion, is that any action (not involving CO2) that changes the pH of the water is generally a result of the change in TDS. Hard water generally has high TDS levels, high pH, high KH, and high GH. Soft water generally is low in TDS, a low pH, KH, and GH. The occurrence of a pH rise, or decline, (not involving CO2) is generally accompanied by an increase or decrease in TDS. It is this change in TDS and the resulting change in osmotic pressure that impacts our fish.

The problem is not a shift in pH; it's a move between hard water and soft water. It's a change in the amount of TDS and this change is better-measured using KH and GH readings than it is using pH readings.

For all of you folks who continue to use salt as a constant tonic, salt is increasing the TDS. A large water change without replacing the salt can cause these same issues (as can refilling a tank and then re-adding salt, in which case you are double whamming your fish), as can adding salt to an aquarium too quickly. This can also impact new fish brought into the aquarium, as they may be accustomed to a TDS level lower than what is in your tank. The pH levels in each of these circumstances may be the same, but the TDS levels will be different, and it is an abrupt change in TDS levels that is the real cause of “pH shock”.

pH effects on other substances:

We are all aware (or at least should be) of the effect pH has on the toxicity of ammonia. At a lower pH, ammonia is converted into ammonium, which is not toxic. As the pH increases, ammonium is converted into ammonia, which is toxic. What is less known is that pH has the same effect on other toxic substances such as chromium, mercury, copper, and iron.

As the pH falls (solution becomes more acidic) many insoluble substances become more soluble and thus available for absorption. For example, 4 mg/L of iron would not present a toxic effect at a pH of 4.8. However, as little as 0.9 mg/L of iron at a pH of 5.5 can cause fish to die.

It is these effects that present the myth of “pH shock”. It is not the change in the pH itself that causes issues thought of as “pH shock”. It is the physiological effects associated with an abrupt change in TDS and/or the synergistic effects of pH increases, or decreases, on toxic compounds within the water.

Evidence of this can be achieved by observing planted tanks with CO2 supplementation. These tanks can experience wide swings in pH over a 24-hour period with no ill effect on the fish, especially for those who turn off their CO2 regulators at night. Why can the pH change in these tanks without impacting the fish? Because carbon dioxide is influencing pH without affecting the amount of TDS in the tanks.

How Does This Affect Us:

Outside of a better understanding of our water chemistry and its effect on our fish, this information, alone, should not provide a bases for changing what we are doing in acclimating our fish and aging our water (when necessary).

My tap water comes out with a pH of 8.4, 3dKH, 4dGH; relatively soft water with a high pH. However, the pH declines to neutral within a short amount of time (a couple of hours at most). The pH declines but the KH and GH do not. This may be because the water company is adding some type of temporary alkali to the water that precipitates out once the water is no longer under pressure. I am currently aging my replacement water overnight to allow the pH to stabilize and match the water in the tank. While I am investigating discontinuing this practice, I will not do so until I have a better understanding of what (if) the water company is adding to the water resulting in this effect. I will not do so until I am able to confirm that the temporary alkali they are (maybe) using is not affecting the amount of TDS in my tanks. While my KH and GH readings do not change, this does not mean my TDS levels are the same. (Remember, aquarium salt will increase the TDS levels of your tank but will not impact pH, GH, or KH. It could be that the temporary alkali being utilized by the water company has a similar effect).

It may also be that my tap water is totally void of CO2 and that it subsequently picks up this compound from the atmosphere, resulting in the drop in pH. In fact, if you use a CO2 calculator and input a pH of 8.4 and a KH of 3dH (my tap water readings), the result is 0ppm CO2. If you then input a pH of 7 with the same KH of 3dH (my tap water readings after it has sat out for a while), the CO2 has increased to 9ppm. If you do the same math, adding 9ppm CO2 to my tap water, then the pH will drop to 7, which is exactly what I am experiencing. If this were the case (and I am seeking information from my water company for confirmation), then it should be perfectly safe to go straight from the tap to my tank even though there is a differential of 1.4pH.

Fact is, something is influencing the pH of my tap water and I do not know what it is. Until I have that answer, I will continue to age my replacement water before use.

For those of you on well water (or even city water, if the water company is adding CO2 to increase alkalinity of the water while it is in their pipes), if the cause of a pH increase (once water is pulled from the well) is CO2 precipitating from the water, then there is no need to age your water before it is added to the tank. It can safely be added straight from the well provided the pH is not to an extreme (for this purpose, I would use a pH of 6.4 as the extreme) and the end result does not constitute a change in pH above 1.3 units (being conservative).

If you're on well water (or city water, if the water company is stripping CO2 to decrease acidity of the water while it is in their pipes), and your water is devoid of CO2, the pH will naturally drop once the water has access to the atmosphere. This type of fluctuation in pH, provided the shift is below 1.7ph units, and you do not have ultra-sensitive fish (Discus come to mind), should not be harmful.

Now, all of this said, there remain pH extremes that can cause “pH shock”. In the wild, fish avoid waters below pH 5.4 and above 11.4. There are studies that indicate exposure to pH in these extremes can cause physiological damage and mortality. However, most environmental studies conducted on the impacts of “acid rain” and agricultural run off have determined that the pH shifts resulting from these events has a devastating effect on plants and microorganisms, which in turn, as each layer of the food chain is impacted, results in a decline in fish population. Again, provided the change is not to the extremes, physiological impacts on the fish, of the pH shift itself, is not the cause of increased fish mortality during these events.

Other Information:

Provided extremes are avoided, fish can adapt to the pH of the water (between 6ppm and 9ppm). Unlike toxins, such as ammonia, nitrite, and nitrate, no evolutionary step is required. Most species of fish simply have to adjust osmoregulation to adapt to the pH (which is really adapting to the TDS levels) of the water we keep them in. If properly acclimated, initially, then keeping our Oscars, JDs, GTs, Convicts, and other SA/CA cichlids in hard water with a high pH is not a concern. However, breeding and fry rearing can be impacted with many species kept outside of their “ideal” range.

Additional References

  • The below text is quoted from this link:

“The effect of transferring hatchery-reared rainbow trout (Salmo gairdneri) from water with a pH of 7.2 to water with pH's ranging from 8.5 to 10.0 was evaluated in 48-h tests. All fish survived in the control (pH 7.2) and at pH 8.5. Survival was 88% at pH 9.0, 68% at pH 9.5, and 0 at pH 10.0. After the 48-h exposure, the remaining test fish were fed their usual pelleted food. Trout in the control and those held at pH 8.5 fed well. Only a few of the fish held at pH 9.0, and none of those held at pH 9.5, fed.”

  • A good link, with some pertinent information in the pH section:
  • The below text is quoted from this article from the University of Florida

“A sudden pH shock may also be harmful, especially to young fish. Within the range of pH 6 to 9, a pH shift of less than 2 units can be tolerated by most fish. If a large difference in pH exists water can be mixed to gradually acclimate fish.”

  • Note: A pH shift of two units would involve an increase from a pH of 6 to a pH of 8 (not like increasing from 7.2 to 7.4).

"What is really interesting is that the acid-base exchange rate is also dependent upon salt (Na+ and Cl-) solution, so GH and KH play a much larger role than may be usually suspected. This thread http://www.fishforums.net/index.php?showtopic=123070 linked to a site whose author deduced this relationship from experience. So, it appears if the salts in the water are favorable, most aquarium fish can adapt to a change in pH pretty quickly – in a matter of minutes really. But, if the changes in salt and total dissolved solids are big, the fish may not be able to use its ability to adjust its pH and that causes shock. Ion exchange at the gills is important for waste removal also."


A follow up to this article is Realities of pH Shock