What Causes High pH in an Aquarium?

Causes and Solutions to High pH in Aquariums

Fish tank with mollies and guppies

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It's not something most of us think about on a daily basis, but almost all liquids, including water, have 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?

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 and moving 1.0 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 your 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, have a very wide range. Do your research thoroughly prior to bringing any species home and make sure your system can accommodate their water quality preferences. Some fish may not be kept in the same tank due to their incompatible pH tolerances.

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, dumps more hydrogen ions into a solution.

Most of the time, high pH originates from your source water. Your initial 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 rather than rely on what the 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 ions 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 you will know exactly how much to add in order 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 dueling 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 changing 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 and causing a "crash" can potentially kill your fish.

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 congruent 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. This is a common issue in many aquascape systems, which should focus on plants and not fish. And a leeching substrate can easily be swapped for a new one that is not changing 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 hardware stores will offer free or discount testing. Persistent issues are commonly resolved by installing a simple RO system. This system will reverse-engineer a pure water source with a neutral pH and limited kH/gH. From this starting point, you will need to correctly calibrate your additives in order to get your pH to the correct level for your fish. Always weigh your additives, rather than measure by volume, in order to get the correct dose.