Realities 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.


Last month I proposed a concept that “pH shock” is a myth. This month I wish to dispel the myth. In reality, pH shock, in large part, does not exist. But it is a myth that dies hard and from reading ongoing threads I find that not many are buying into the concept that pH has little to do with it. For circumstances normally diagnosed as “pH shock”, the real problem is osmotic shock, which results from an abrupt change in TDS levels (Total Dissolved Solids), or the increased toxicity of chemicals (or heavy metals) that can occur following a change in pH. But, unless the extremes are avoided (for our fish, let's consider the extremes below 6pH and above 9pH), pH itself is not to blame.

pH Shock is a Myth

In a Myths of pH Shock, I identified that I had discontinued the practice of aging my tap water for all tanks, excluding the Oscar tank. In reality, well prior to posting that article I had completed my research, receiving confirmation from my water company that the high pH values from my tap are a direct result of their intentional removal of CO2 from the water (but the article was written, so I submitted it as is). Once this confirmation was received, I put theory to practice and have been performing water changes straight from the tap on all tanks for several months, with no ill effects on the fish.

Water straight from my tap registers about pH 8.4. Once the water is released from the pipes it quickly absorbs CO2 from the atmosphere and drops to around 7.0, where it is stable until the next water change increases it (temporarily). Keep in mind, for my Oscar tank, a 100% water change is normal, as I reset nitrates to zero each week.

Below is a pic comparing pH values. The pic defines, from left to right: pH values of my tanks before a water change pH value of my tap (using the low range pH kit) pH value of my tap (using the high range pH kit) pH value of my tap about 1 hour after the water change.

As you can see, there is a significant difference, more so than would commonly be acceptable under risk of pH shock. Immediately following a water change (resetting nitrates to zero), my pH registers above 8 (CO2 is obviously being absorbed as the tank is being refilled.) Within an hour, it has dropped substantially, and by the following morning it is down to 7. To state the obvious, this is a significant bounce up and then back down. Yet, I have not experienced “shock” of any nature because, by and large, pH shock does not exist. The real culprit would be an abrupt change in TDS values. Since CO2 fluctuations do not result in changes in TDS values, the danger of “shock” is virtually non-existent. And since the TDS values of my tap water do not change (only the pH does), no shock has occurred.

Why is pH Blamed?

The other danger with pH fluctuations of this nature is that pH does influence the toxicity of compounds (such as ammonia) and metals (such as iron, copper, mercury, and aluminum.) Before placing this concept into practice (by discontinuing water aging) it is important you have an understanding of what is in your tap water. A few basic concepts of understanding are important:

  • Heavy metals generally become more toxic in lower pH. As the pH decreases, these products become more soluble, making them more available for absorption by the fish. However, this fact should have little influence on a decision to age your water. If the synergistic effects of pH on heavy metals are a concern, aging your water does not eliminate this concern as the metals will remain, presenting the same problem whether or not the water is aged. It does not matter if the pH drops in a water aging container or in the main tank. The metals will remain just as toxic.
  • Ammonia becomes more toxic as the pH increases. This is of significant concern but only applies to a cycling tank, a tank suffering “Old Tank Syndrome”, or if you register ammonia in your tap water (more on this later).
  • There is a relationship between calcium and pH that influences the effects of pH on fish. If you have extremely soft water (general hardness), or use R/O water, and there is a significant difference in pH between replacement water and tank water, then it is best if you continue to age your water. It should be noted that I have “soft water”, but not “very soft”. A general rule would be that if your GH readings fall below 4DH (70ppm), and there is a significant difference between tap pH and tank pH, you should continue to age your water.
  • The general rule is that you wish to prevent a pH change of greater than 1.7pH units. As an example, a pH shift from 7.0 to 8.7 would be a change of 1.7pH units.

While I consider pH shock a myth, pH does have physiological effects on fish. However, most fish appear to be indifferent to pH within an approximate range of 5.5 to 10.5. In general, there are little physiological differences to the same fish kept between these pHs. The physiological effects of pH on fish are only found when the fish are subjected to extremes.

A few other facts concerning pH:

  • Alkaline pH can increase the mortality of incubating eggs of some species. This is likely because acidic waters are somewhat bacteriostatic.
  • For most fish evolved to exist near neutral or acidc pH, at high pH (above 10), gill mucus cells and opithelial cells are hypertrophic (increase in size), which impacts osmoregulation. Corneal damage may also occur.
  • “Blood pH” is the most often quoted physiological cause of pH shock. While it is true that terminal physiological measurements on adult fish succumbing to low pH (in soft water) indicate the cause of death is iono-regulatory failure (inability to maintain blood plasma ion levels (electrolytes, sodium, and calcium), this occurs at a pH of about 4.5. A pH this low is not something we would not normally find in our tanks, so it can be discarded as the physiological “cause” of pH shock.

Water Aging

Like so many myths, the myth of pH shock has some bases in fact. But for pH Shock to exist, there has to be a physiological impact and I am simply unable to find one except in extreme conditions not normally found in our aquariums. Combine this with the fact that I have placed theory into practice, by discontinuing the aging of my water to match pH values of the tank (which is only allowing the water to absorb CO2), with no ill effects, and you can only come to the conclusion that many of us go through the hassle of aging water unnecessarily. If you are keeping “non-sensitive” fish and have no other issues with your tap water, but are aging your water simply to stabilize the pH, chances are this is not a necessity.

For the most part, if eliminating water aging, it would be considered safer to have water where the pH comes from the tap high and then declines, than to have water where the pH comes from the tap acidic, and then increases. Tap water that registers for ammonia presents some complications as well. Ammonia becomes more toxic as the pH increases. If your tap water contains ammonia and has a high initial pH, then declines, the ammonia will be more toxic as it is added to the tank (as opposed to allowing the pH to decline in an aging container).

Aging your water does have other benefits.

  • It allows chlorine and other potentially harmful gases to dissipate naturally.
  • For planted tanks it allows water devoid of CO2 to absorb CO2 from the atmosphere.
  • It allows water low in dissolved oxygen to absorb oxygen from the atmosphere.
  • It allows pre-filtering to remove sediments and potentially harmful substances
  • It allows iron particulates that may be present in well water to settle into the bottom of the aging container.(A very common issue with well water)

If you wish to discontinue aging your water it is necessary to first understand what is in your water and then determine if changes in pH will affect those substances. I've provided some guidelines to this effect above. You can also “test” the theory using a small (cycled) tank and a fish that is of minimal importance, perhaps one you purchased just for this purpose. However, if you take this route, give the fish a month and confirm its health prior to undertaking any testing.

It is also necessary to test the oxygen content of the water. Another common “mistaken diagnoses”, commonly attributed as “pH shock”, is that some water sources will be as devoid in dissolved oxygen as mine is in CO2. Fill a tank up with hypoxic water and you can wipe out a tank of fish.


In closing, different fish may react differently to pH flucuations. I would not attempt this (without further testing) with sensitive fish, such as discus, cardinal tetras, wild caught fish, or even neon tetras (as well as other “sensitive” fish.) However, I can state with certainty, the fish I own are not experiencing any issues. That list is as follows:

  • - Oscars
  • - Convicts
  • - Firemouths
  • - Angelfish
  • - Common Pleco
  • - Giant Danios
  • - Silver Dollars
  • - Serpae Tetras
  • - Pristila Tetras
  • - Red Tailed Black Shark
  • - Bronze Cory
  • - Green Cory
  • - Albino Cory
  • - Kissing Gourami
  • - Guppies
  • - Platies
  • - Swordtails
  • - Betta
  • - White Cloud Mountain Minnows