What Causes High pH in an Aquarium?

Causes and Solutions to High pH in Aquariums

Fish tank with mollies and platys

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It's not something most of us think about on a daily basis, but almost all liquids, including water, are either acidic or basic, which is measured as a pH level—but in aquariums or fish tanks, that pH level can have a huge impact on your fish and plants. High pH in aquariums happens, but there are ways to manage it.

What is pH?

The letters pH stand for the "power of Hydrogen." Since capital H is the chemical symbol for hydrogen, it is always written as "pH." The pH is a measure of hydrogen ions in a solution. It is calculated as the inverse, base-10 log of the total concentration of hydrogen ions. Therefore, solutions with higher hydrogen ion concentration will have a lower pH (acidic) and less hydrogen ions will make for a higher pH (basic) solution. The pH scale goes from 1 to 14, with pH 7.0 being neutral (neither acidic or basic). Moving 1.0 unit in the pH scale, such as from 6.0 to 7.0, is a 10x change in the hydrogen ion concentration.

Why does the pH of aquariums matter?

Hydrogen ions play many important roles in transport across membranes. In fish, this is critically important at the gill-water interface. Given that hydrogen is a positively charged ion, there are many potential bonding pairs which can interfere with ion transport. Disruption to this process can have severe consequences to the health of your fish if they are unable to offload waste from their blood.

Depending on the species, your fish may have a wide pH tolerance or a very narrow one. Many marine fish species are known for very small pH tolerances, whereas other fish, such as koi and goldfish, can tolerate a very wide pH range. Do your research thoroughly prior to bringing any species home and make sure your aquarium can accommodate their water quality preferences. Some fish species may not be kept in the same tank due to their incompatible pH tolerances. Examples of incompatible fish species are discus cichlids, which require soft water with low pH, that would not do well with African cichlids that require high pH and hard water.

Common Causes of High pH

Many biological processes can interfere with your aquarium’s pH. Decreases in pH are more common, since more activity, including algae, fish and invertebrates, introduces more hydrogen ions into a solution.

Most of the time, high pH originates from your source water. The initial water pH will depend on your location and any filtration you have in your home. If you are using additives to manipulate your pH to accommodate certain fish species, it is critical that you do your own tests to determine dosage rather than rely on what the pH adjusting bottle prescribes, unless you are starting with RO water. Depending on the chemical makeup of your tap water, your buffering additives may react differently depending on what cations and anions are present in various concentrations. A simple test is easy using a known volume of water and a known weight, not volume, of your additive. Test your pH and kH prior to mixing and after. Now, based on your test in a known volume of water, you will know exactly how much to add to your aquarium to set your pH just right.

The second most common cause of high pH is lots of plant and/or algae activity. Plants and algae survive on the alternating processes of photosynthesis and respiration. Respiration in plants and algae occurs in the absence of sunlight and is the same as most aerobic organisms on earth. Summarized, cellular respiration uses oxygen and fuel to create energy or heat, carbon dioxide, and water. In the reverse, photosynthesis uses light energy to take carbon dioxide and water to make oxygen and fuel for the plants to survive. By removing the carbon dioxide from the environment, and not increasing the carbonate alkalinity, your pH can rise slowly or dramatically, given the total amount of plant life in your pond or aquarium.

Various substrates may also be leeching cations into your water and bringing up your pH. Be sure that you are using an appropriate substrate for your species and tank setup. A simple test can be done by soaking some substrate in a bucket of freshwater and measuring the change in pH.

Solutions to Correct pH


If the pH of your aquarium is wrong, DO NOT attempt to correct it quickly. pH changes must be made gradually so your fish can slowly acclimate to their new environment. Shifting the pH too quickly, either up or down, can potentially kill your fish. Changes should be no more than 0.3-0.5 pH units per day.

Resolving high pH requires correct identification of the originating cause. High pH in your starting source water may be resolved through heavy filtration or a water softener, in cases of high pH with concurrent high kH and/or gH. Bounding pH from heavy plant concentrations may be deduced from hourly pH checks during daylight hours. If your plant load is too heavy, the simple fix is to remove some of the plants or reduce the lighting. This is a common issue in many aquascape systems that focus on plants and not fish. And a leeching substrate can easily be swapped for a new one that does not change your water pH.

If you are sure your source water is correctly calibrated and your plants are not overwhelming your system, you may benefit from a more thorough evaluation of your water source. Many small levels of various cations and anions may significantly impact your final aquatic environment. Some cities or pet stores will offer free or low-cost water testing. Persistent issues are commonly resolved by installing a simple RO system. This filtration system will produce a pure water source with a neutral pH and limited kH/gH. From this starting point, you will need to correctly measure your additives to get your pH, kH (alkalinity) and gH (hardness) to the correct levels for your fish. Always weigh your additives, rather than measure by volume, in order to get the correct dose.