Aquarium Water pH Maintenance

Test Strip for pH

JazzIRT / Getty Images

What Is pH?

The term pH stands for "power of Hydrogen," and since "H" is the atomic symbol for the hydrogen element, the "H" in pH is always capitalized. The pH is the acid-base balance of a solution and is measured in a range from 1 to 14.

Water, or H2O, is composed of hydrogen and oxygen atoms. Neutral water contains equal amounts of hydrogen ions (H+) and hydroxide ions (OH-) and is given a pH value of 7.0. Dissolved chemicals and minerals in water can change the balance of those ions from a neutral state to be acidic if there are more hydrogen ions than hydroxide ions, or basic if there are fewer hydrogen ions. Acidic solutions have a pH value of less than 7.0, while basic solutions have a pH value of more than 7.0. The further these values decrease or increase from 7.0, the more acidic or basic (respectively) the water becomes.

What Is Normal pH?

There is no "normal" pH that applies to all fish. Because fish originate in ponds, rivers, streams, lakes, and oceans that have different pH levels, the optimum pH levels for fish varies by species. Saltwater fish prefer a basic pH of 8.0 or above. African cichlids often come from lakes that have a pH value above 8.0. Tropical fish from the Rio Negro in Brazil may live in acidic water with a pH of 5.5 or lower.

Keep in mind that pH is not static; it changes over time. In fact, it may even change over the course of a single day. In nature, due to plant respiration and photosynthesis, pH typically drops at night and rises during the daytime. The pH may change as new fish are added or removed, as water is added or changed, and as the biological processes change in the aquarium.

Preferred pH of Common Freshwater Fish

normal ph levels for aquariums illustration

Illustration: The Spruce / Martisa Patrinos

How Important Is pH?

Significant pH changes are particularly hard on young and sick fish. In a number of species of fish, breeding occurs only within a specific pH range.​

If you are planning a new aquarium it's wise to know the pH of your water source, so you know beforehand if it is compatible with the species of fish you want to keep. Some fish such as Discus, and certain other cichlids, thrive in very narrow ranges of pH, which should be taken into consideration when setting up their aquarium.

When moving fish from one aquarium to another it is important to match the pH levels. Sudden changes in pH account for many fish losses that occur when fish are brought home from a pet shop. Neon tetras are particularly sensitive to sudden changes in pH, and can easily be shocked when moved.


Changes in the pH, especially sudden changes, can prove harmful or even fatal to fish. As the pH rises, it increases the toxicity of chemicals such as ammonia. It is an important factor to monitor while breaking in a new aquarium.

How Often Should I Check pH?

The pH should be tested at least once a month, though preferably every two weeks, to allow for the detection of trends before they become a problem. Keep test results in a logbook for future reference. Remember that because pH can vary based on time of day, testing at different times of day can yield different results even though nothing is wrong. For this reason, testing should take place at the same time of day, preferably in the afternoon.

Any time there is a fish illness or death, the pH should be tested. If the tank is treated with medication, the pH should be checked when treatment is begun, on the final day of treatment, and again a week later. Perform water changes as needed when the pH starts to vary from the optimum range for the fish.

It is also wise to test your water just before purchasing new fish. Check with the shop where you are purchasing the fish to see what their water pH is. It's important that the pH of water the fish is currently in is not significantly different than the pH of your water at home (preferably within 0.2 units above or below the home pH value).

Should pH Be Altered?

I recommend sticking to the axiom of "if it's not broken, don't fix it". Don't spring into action simply because the textbook says the optimum pH for your fish is 6.4. and your water tests out at 7.0. As long as the pH is stable, and the fish show no signs of distress, it's best to leave the pH at the level of your local tap water. Also, most aquarium fish sold today are raised in fish farms that do not keep the fish in the pH of the natural habitat water. So, a pH of 6.8-8.0 is a safe range for keeping most freshwater fish.

If the fish are not thriving, or if testing shows that a trend is occurring, such as a steady drop or rise in pH, the problem should be addressed. Pet stores sell commercial products designed to raise the pH or to lower it, if necessary, to adjust the pH of your local tap water. Proactive water care is always your best bet. Performing frequent partial water changes, and vacuuming the gravel are the most important things you can do to keep water pH stable. Over time, the biological filter bacteria that break down fish wastes will utilize the alkalinity (carbonate) in the water and the pH will gradually drop (become more acidic). You can prevent this by doing water changes to remove the lower pH water and adding fresh, dechlorinated water that has higher alkalinity to raise and stabilize the pH level.

The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Aquarium Water Quality: pH. Florida Department of Agriculture and Consumer Services.

  2. pH. United States Environmental Protection Agency.