Aquarium pH: How to Safely Lower or Raise pH Levels

Understanding which species of fish are suitable for your aquarium begins with knowing the pH levels of your tap water. Additionally, you may need to modify the pH levels in your aquarium if you think they are too high or too low for the species of fish you’d like to accommodate.

To determine the right method to adjust your aquarium pH levels, you will need to fully understand what can cause an increase and decrease in pH. Otherwise, you may end up fighting an uphill battle.

What is pH

The acronym pH stands for the potential of hydrogen or the power of hydrogen. It is a scale for measuring the acidity or basicity of liquid solutions that was created by Danish chemist Soren Sorenson in 1909.

The pH Scale

What is a pH scale

On the pH scale, which ranges from 0 to 14, lower numbers below 7 indicate acidic solutions, higher numbers indicate basic solutions, and 7 is regarded as neutral which falls into neither the acid nor basic categories, like pure distilled water.

Since the scale is logarithmic, each entire pH number signifies a change of 10 times the preceding value. A pH value of 5 is 10 times more acidic than a pH of 6, as lower values denote increased acidity, whereas numbers over 7 denote an increase in basicity.

pH of Water

Source of WaterpH Value
Rainwater5.0 – 5.8
Reverse osmosis water6.0 – 6.5
Distilled water7.0
Demineralized water7.0
Pure water7.0
EPA guidelines for pH of tap water6.5 – 9.0

Measuring Your Aquarium pH Levels

There are several reliable methods capable of measuring the pH of your aquarium’s water, but the accuracy of each method will vary. They are as follows:

  • Annual municipal water report
  • pH meter
  • Solution-based test kit
  • Litmus paper

The quickest approach to learning the pH level of your home’s tap water is a quick Google search of: (“My City” drinking water annual report). The majority of municipalities publish an annual report on drinking water that includes hundreds of various parameters, including those related to bacteria, minerals, ph, and more.

Cities and even specific areas within a city will have pH readings that are very different from one another. My city’s annual report, for instance, shows that the pH of my tap water has been adjusted to a level of 7.65:

Measuring Your Aquarium pH Levels Report

An aquarist may easily and accurately measure the pH levels in their aquarium with a digital pH meter. Enabling you to regularly check the pH of your aquarium and promptly identify any changes in pH that could be harmful.

Although solution-based testing is the most reliable procedure, it may not always be as accurate as a pH meter. Since a digital pH meter must be calibrated, it can occasionally lose calibration without the user’s knowledge.

Litmus paper is a single-use, disposable test kit that is one of the more popular ways to determine the pH level of an aquarium. Primarily because they measure numerous water parameters simultaneously in an all-in-one package.

Proper pH for Freshwater Aquariums

The majority of freshwater fish species will tolerate a pH between 6.5 and 7.5, African cichlids prefer a higher pH between 7.8 and 8.5, and Amazonian fish prefer a lower pH between 5.5 and 6.8.

However, some species of fish can be rather sensitive to varying water parameters. For more information that is specific to each species, we advise looking through our freshwater fish database.

How to Raise pH in Aquariums

The best method to raise pH in aquariums will entirely depend on the starting pH levels; picking the right strategy will require using a method that can raise pH levels above your existing levels.

If you are having trouble raising the pH in your aquarium, you may have low levels of KH; since KH serves as a buffer, you might need to take care of this issue first if you do. For more details, view our guide on buffering capacity (KH).

While aquariums are operational, pH levels are gradually decreasing over time. The nitrification process itself is primarily responsible for this, which involves the breakdown of ammonia and nitrites that causes bicarbonate to be destroyed and results in the release of carbonic acid. The rate of this process is determined by how much fish, uneaten food, or other decomposing material is present within your aquarium.

Effective methods to raise pH in aquariums are as follows:

Water Change

Depending on your aquarium’s stock levels, water changes of 20 – 50 percent each week will mitigate decreasing pH levels by removing a percentage of the carbonic acid and replacing it with fresh water with higher levels of bicarbonate.

During your weekly water changes, siphoning the substrate’s surface will remove the build-up of excess organic waste and food. This method is best used to prevent pH levels from falling well below your initial tap water’s pH.

Surface Agitation & Aeration

Smaller air bubbles are more effective at transporting and releasing carbon dioxide, which is removed via aeration. For an aquarium with plants, higher carbon dioxide levels are fantastic, but in tanks without plants, the buildup of carbon dioxide will gradually lower pH over time.

Overfilled fish tanks frequently result in poor aeration since the filter cannot effectively break the water’s surface. This issue can be swiftly resolved by lowering the water level in your aquarium. If you think your filter isn’t agitating the water’s surface sufficiently, another option is to add an air stone.

Distilled & Demineralized Water

Distilled and demineralized water are capable of raising the pH values of aquariums to a neutral value of 7 pH. A 70% to 30% tap water mixture is the most effective way to dilute tap water for aquarists who have to deal with excessively low pH values in their tap water.

Calcareous rocks

Any rock or stone containing calcium will cause the pH to rise; these rocks or stones are typically softer than other kinds of rock. Calcium carbonate, which can raise pH to the desired level, is primarily found in limestone, seashells, marl, and dolostone (dolomite). However, only add modest amounts while continuously checking the pH level.

The two most effective rocks for raising pH are limestone and dolostone, which will also slightly increase GH and KH. It’s more effective by placing one or two small bags of crushed limestone or dolostone (dolomite) into your aquarium filter which can easily raise the pH from 5.0 to 6.0 or 7.0. Test the water frequently until you achieve your desired results; if the pH seems to be rising too high or not high enough, add or remove some.

Compared to other rocks, seiryu and alabaster rock contain fewer calcium-based minerals. However, they are perfect for the majority of aquarists who want to gently raise their pH levels. Untreated rock has a greater capacity to increase the pH of an aquarium than acid-washed rock since acids will dissolve calcium carbonate.


Substrates containing calcareous rocks such as limestone, dolomite, and crushed coral will also increase pH levels. River rocks and most other varieties of aquarium substrates purchased in stores have relatively little impact on pH levels since they are neutral-based.

How to Lower pH in Aquariums

Lowering pH to the neutral range is rather easy, but finding ways to maintain an acidic pH value can be a little more challenging.

If you are having trouble lowering the pH in your aquarium, you may have high levels of KH; since KH neutralizes weaker acids, you might need to take care of this issue first if you do. For more details, view our guide on buffering capacity (KH).

Effective methods that can safely lower pH are as follows:

Reverse Osmosis Water

The pH of tap water can be lowered to a range of 6.0 to 6.5 using reverse osmosis filtration devices. The best way to dilute tap water for aquarists who are coping with abnormally high pH levels is a 70% RO and 30% tap water mixture.

RO units filter out 99% of total dissolved minerals that are primarily responsible for increasing the levels of pH within your water source. As opposed to other methods, RO units remove the problem right from the source, so you won’t need to worry about your pH levels fluctuating.

All species of fish do require certain minerals to survive; 50% to 70% RO mixtures will suffice, depending on how high your initial pH is. Never use 100% RO water to fill an aquarium since it will virtually eliminate all minerals and leave you with very little buffering capacity (KH), which could cause your pH to crash and fluctuate drastically.

Distilled & Demineralized Water

Distilled and demineralized water are capable of lowering the pH values of aquariums to a neutral value of 7 pH. A 70% to 30% tap water mixture is the most effective way to dilute tap water for aquarists who have to deal with excessively high pH values in their tap water.

Aquatic Plants

Adding plants into your aquarium without a CO2 system mildly reduces pH levels. With a CO2 system, pH values are capable of decreasing significantly depending on the number of plants and the amount of CO2 used.

In general, plants require more CO2 to flourish and create organic waste, which effectively lowers pH by increasing the load on the nitrification process. Higher levels of dissolved CO2 produce carbonic acid and decreases the pH value.

Tannic Acids

Tannic acid, which has a pH base value of 6.0 and cannot lower pH below its base value, is produced by all of the methods listed below. In hard waters with higher KH, tannic acids have a lesser impact on pH.

Peat Moss & Sphagnum Moss

As minerals are absorbed, peat moss and sphagnum moss releases gallic and tannic acids, which have the effect of lowering pH, GH, and KH. Peat moss can also discolor water; to lessen the impact of staining, soak it in a bucket of water for 24 to 48 hours.

Peat moss and sphagnum moss is the most effective method of maintaining a lower pH since it produces numerous acids, capable of lowering pH to 4 – 6. Place one or two small bags of moss into your aquarium filter for quick results which will slowly decrease pH levels over time. Test the water frequently until you achieve your desired results; if the pH seems to be dropping too low or not low enough, add or remove some.

However, soaking peat moss for an excessive amount of time may reduce its impact on pH. And lastly, a carbon filter will also be helpful because it will remove any tannins that might contribute to discoloration.

Driftwood & Bogwood

Driftwood and bogwood release tannic acid which will typically reduce an aquarium’s pH value by a little to a moderate amount, depending on the amount that you use and how hard your water is, to begin with. The effect that bogwood and driftwood have on pH gradually lessens as it ages in your aquarium.

Treated or boiled wood will reduce its impact on pH, as this process will cause the release of the tannic acids. Tannic acid, which is released by driftwood can also result in light brown discoloration in the water.

Aquarium Safe Leaves

Almond, beech, oak, and catappa leaves, which also release tannic acids, are a great way to both lower the pH of the water in your aquarium and give a natural-looking accent to your aquascape. A few pieces at a time should be added to avoid overburdening the nitrification process.


Pumice, volcanic soil, peat moss, and compost are substrates that will lower pH levels to a minor extent; these substrates are generally used in aquariums to support aquatic plants. Compost and peat moss are the two most effective methods capable of reducing pH to 4 – 6, depending on the amount that you add.

Stabilizing pH in Aquariums

Consistency is key. Scheduling weekly or biweekly tank maintenance and water changes without deviation will be the main strategy for maintaining consistent pH levels. The water’s parameters are slowly but steadily changing between each water change.

Every time you decide to add or remove something that has the potential to change the pH over time, do so in gradual increments and monitor the pH level carefully. When adding any solutions to an aquarium, inquire if they could potentially change the chemistry of your water. This is especially true with medications and other aquarium solutions that contain substances like formaldehyde or methanol.

A reasonable rule of thumb is that anything that is mineral-based, like rocks, will likely increase pH whereas anything that is organic-based will likely cause the pH to decrease to a certain degree.

You may have extremely low or high levels of KH if your pH is sporadically fluctuating or if you are having problems raising or lowering pH. If your pH is continuously decreasing or is very low, to begin with, your source of water may have very low levels of KH.


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