pH Scale is logarithmic. This means that there is a x10 change in acidity or alkalinity for each change of 1 unit in pH.
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pH Scale


The scientific definition of pH is “the negative logarithm of the hydrogen ion (H+) concentration.” This is where the H in pH comes from. The lowercase letter “p” refers to “power” or exponent.

As minerals dissolve in water the equilibrium shifts. An increase in the amount of hydrogen ions makes water more acidic, while a decrease makes it more alkaline.


ph scale

The pH scale is logarithmic. This means that there is a x10 change in acidity or alkalinity for each change of 1 unit in pH.

Thus pH5 is 10x more acid than pH6, And pH5 is 100x more acid than pH7.


pH scale consists of numbers from 0 through 14, with 7 being Neutral, that is used to indicate the acidity of a waterbody.

The pH is often misunderstood. A pH of 7.0 may be perfect, according to many, but it may be unsuitable for fish if there is a lack of calcium or buffering capacity. pH should be seen in context with all the constituents of a water sample. It is not so much the pH itself that is crucial, but the ionic content giving rise to the pH. It is only at the extremes that pH is a true indicator of real trouble.

At low pHs the toxicity of ammonia is low. Low pHs may be brought about by carbon dioxide, produced by animals and by plants at night, dissolving in water and forming carbonic acid.

If water of a higher pH is added then there may be a sudden increase in pH. Associated with this rise will be a rapid increase in the toxicity of any ammonia present.



pH Crash: Acidosis

Water is constantly rushing (leaking) into the fish through their skin and gills. The kidney is responsible for pushing that water out. Water with a low pH scale is getting into the fish, and this requires that the body mobilize its own natural buffers to sustain the blood pH. Quickly, these buffers are exhausted and the blood stream suffers a low pH called acidosis. Acidosis can be terminal for Koi or Pond fish.


If you notice your fish look like they have a white haze on their skin or that their skin is peeling...Test for pH and do an immediate 50% water change before going any further.

Think of pH as the way the water feels against the fish’s skin. A low pH “burns” the skin because it is acidic. High pH “chaps” the skin because it is alkalinic. The ideal pH reading for Koi and goldfish is around 7.5 but these fish can tolerate a range between 6.8 - 8.2.

We prefer to use pH strips to test pond water because not only does it give you the pH level it also indicates whether the water is buffered properly or not. Buffered water means the pH is stable.

pH tests vary a few points at different times of the day. It tends to run higher at night then lower by morning, taking it’s cue from the natural CO2 and Oxygen exchange of the plants in the water. The difference in the pH at different times of the day is called pH swing. When it is more than just a couple of points off it can be dangerous and unpredictable. The object is to stabilize the pH at an acceptable level.

Now think of the pH suddenly falling. It would feel sort of like being dropped into a vat of acid. When the pH suddenly falls to a 6.5 or below it is called a pH crash and is very deadly. Fish subjected to a mild pH crash suffer peeling of the skin, very similar to our sun-burned skin peeling off. More signs your Koi are suffering from a pH crash are: the fish will lose their appetites, lie on the bottom of the pond and show signs of red streaking in the fins and body.

Sever crashes can end with complete fish loss.

A drop in pH in a pond to below 7 will lead to sudden changes in your Koi's Health. The longer the pond's pH stays in this range the worse your Koi's health will be. Colors in your Koi are likely to fade through the deposition of excess mucus while in extreme cases, Koi may even start to grasp at the surface of the pond. Acidic pond water is also likely to be destructive to plastic and metal surfaces in turn this will cause the pond water to become more toxic.

In the case of an emergency a water change is in order to take the pH up quickly. A very clean system with little organics tends to be lower in pH. Bead filters, because of their efficiency, can actually contribute to low pH readings. For readings that tend to stay low, baking soda in small quantities can be used to bring the pH up but you do not want to raise the pH quickly unless it’s an emergency because it can shock the fish. Instead, it is best to use carefully prescribed products designed to raise the pH over a period of time and hold it at the desired level. We use pH stabilizers with great results. It’s important to know that the stabilizers must be added back whenever water changes are made.

Concrete leaching into the water will cause high pH readings. Newer ponds where stones have been concreted together in a stream or waterfall often see readings close to 10.0. Again, care must be given to bring it down slowly. Once it is down you will then stabilize it with pH stabilizers.

Aquariums and ponds are not immune to pH swings and crashes. It’s a great idea to test in the morning and in the evening for a week to get an idea of the severity of any swings in the readings.

Increasing the pH is usually as simple as adding common baking soda slowly to the water.



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If you find that your pond is subject to pH swings there are some very simple (and not really pricey) things you can do to lower, raise and buffer your water, listed below are 2 such tests to keep on hand. Testing is the key and stabilizers are the answers.


ph scale pH Up Liquid
~Price $16.99 16 oz. bottle will treat 2,400 gallons



ph scale pH Down Liquid
~Price $16.99 16 oz. bottle will treat 4,800 gallons











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When KH is depleted, it causes pH to fall or crash into the acidic range. KH MUST be replaced so the pH Scale will move back into the alkaline range. WARNING:

  • NEVER correct pH without testing ammonia which should be zero. If not, add Ammo Lock to bind the ammonia.
  • Now KH (carbonates) can be brought up by adding baking soda, 1/4 cup per 1000 gallons. Circulate through the filter and retest until pH Scale reaches a safe range.







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