Every hobbyist aims to establish a successful aquarium. Fortunately, there is ample information available on various water and tank parameters, fish care, etc., which becomes handy. However, each of us discovers something unique about fish-keeping with experience.

The pH value of aquarium water plays a vital role in keeping your fishes healthy. An incorrect or fluctuating pH can prove fatal for your prized fish collection. It gets impacted by various factors, a few of which are unavoidable.

This article explains what pH is in a fish tank, how it impacts the tank dwellers, and how to balance it.

What is pH in a Fish Tank

The words pondus Hydrogeniiin Latin mean the weight of hydrogen. It is commonly known as the power of hydrogen or pH. Only substances having a water component have a pH value.

A pH value indicates if a solution is acidic, alkaline (also known as basic), or neutral.

The below diagram depicts a pH measuring scale.

What is pH in a Fish Tank
Source Info: elmhurst.edu
  • A pH grid is from 0 to 14.
  • H+ stands for hydrogen, and OH- (minus) represents the hydroxyl ions.
  • A pH of 7 is neutral, where both types of ions are equally present.
  • As the hydrogen ions increase, the pH value drops below 7, and a pH of 0 to 6 represents acidic water.
  • pH values from 8 to 14 have more hydroxyl ions and represent alkaline/basic water.
  • Although the power of both ions is negative, it is generally written as positive (like 105 instead of 10-5) and not mentioned with pH values (like pH = 5).

Below are the pH values of a few common items and the impact of some low pH values on aquatic life.

PH values and Impact
Source Info: usgs.gov pH Scale

Author’s note: Alkaline water refers to a higher concentration of hydroxyl ions, whereas alkalinity of water means the ability of the water to set off acidic buildup. Basic water is represented by higher pH values, whereas alkalinity is measured by the KH (carbonate hardness) or GH (general hardness) of water in parts per million (ppm).

Importance of pH

Recreating a habitat that best resembles the natural environment of your pet fish is the key to keeping them happy, healthy, and safe in captivity. It also promotes natural breeding. The pH levels for freshwater and saltwater aquariums differ.

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A study indicates that most fishes have a pH between 7.7-8.0 in their blood. A few species have it from 7.3 to 7.6. Aquarium water that matches their internal pH level is best.

What Causes pH Drop in an Aquarium

Every fish tank tends to become acidic with time due to:

  • Bioloads/wastage from fishes.
  • Breathing of fish releases carbon dioxide (CO2) and reduces oxygen (O2).
  • Chemicals released by decaying organic matter (like uneaten food, dead leaves, dead fish, etc.)
  • Aquarium plants use CO2 and release O2 during photosynthesis in sunlight, which increases pH. However, there is no photosynthesis at night due to no sunlight. Hence aquarium plants breathe in oxygen and release CO2 at night, lowering the pH.
  • Having no/fewer plants in the aquarium can drop the pH.
  • Overcrowded aquarium.
  • If the replacement water has a lower pH, it will drop the pH after every water change.
  • Gradual release of tannin (a chemical) by driftwood.

Symptoms of Low pH in an Aquarium

Fishes have a mucus/slime layer on their bodies, which reduces friction while swimming and makes it difficult for bacteria/parasites to latch on their skins. However, excessive mucus can be detrimental to their health.

Indicators of a low pH are:

  • Excessive mucus on the bodies of the fishes.
  • Stressed fishes with pale/faded colors. They will find it challenging to swim, eat, or even breathe.
  • Fishes will become lazy and lie in a corner in the tank bottom, or keep swimming to the top level in a restless state.
  • Algal growth. Excessive algae growth is harmful.
  • A drastic drop in pH (more than 0.5) in 24 hours can prove fatal for fishes, even if the overall pH is within the acceptable range.
  • Dirty tank. Unclear water. The decaying matter at the tank bottom/in the tank decor may form sludge.
  • A dirty tank becomes a thriving ground for bacteria that can attack sluggish fishes with lower immunities.
  • All fishes will die at a pH of 4.2. Fish eggs cannot survive at a pH of 5.5.
  • Fishes stop breeding. Refer to the image in section 1.0.

How to Raise pH in an Aquarium

Increase the pH gradually to avoid shocking your fishes. Unless the root cause of low pH is identified and fixed, the tank water will keep moving back to lower pH values.

  • 10-15% weekly/fortnightly water changes (partial) will reduce acidity in water.
  • The replacement water source should not have low pH. Use remineralized RO water or dechlorinated tap water after testing.
  • Avoid overcrowding and overfeeding. It will reduce CO2, nitrate, and ammonia build-up, besides minimizing uneaten food piling at the tank bottom.
  • An efficient filtration system will remove uneaten food and floating matter. An air stone and pump will keep the water oxygenated, raising pH.
  • Add aquatic plants to raise the pH in the aquarium. Refer to section 3.0 for details.
  • Cleaning the substrate and decor periodically (not thoroughly). Removing dead fish, decaying leaves, algal growth, etc., at the earliest.
  • Rocks like crushed corals, limestone, and aragonite-sand substrate increase pH in the aquarium. All these are rich in calcium and release it gradually. Replace the substrate/rocks once they deplete in calcium content.
  • Place some crushed corals/limestone chips in a mesh/cloth in the filter for quicker results.
  • Add one teaspoon of baking soda (sodium bicarbonate)/ per 5 gallons of water. Do it slowly and regularly, as its impact is temporary. It will improve the pH by one point.
  • Use pH up/down chemicals from the market. Regular use of chemicals in a fish tank is not advisable. Use this option sparingly.
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Causes of High pH in an Aquarium

  • If the replacement water contains heavy minerals, it will raise the pH. Check the chemical composition of the replacement water periodically to identify any changes.
  • Adding untreated replacement water from ground sources (like wells).
  • Excessive aquatic plants can increase CO2 drastically at night. Plants should be proportionate to the tank size and the number of fishes in the tank.
  • Excess of calcium-rich stones/substrate. Reduce the number of rocks or change the substrate.
  • Overdose of chemical filters, CO2 injections, or fertilizers can lead to chemical imbalance and high pH in the aquarium.
  • Excess baking soda in the fish tank.
  • Faulty/ineffective filtration system. It should be apt for the tank size and cleaned/maintained periodically for the best results.

Symptoms of High pH in an Aquarium

  • Alkaline aquarium water will break the mucus layer on the fishes’ bodies, making them vulnerable to pathogens.
  • Chemical reactions due to High pH may increase the toxicity of ammonia in the aquarium. It may result in an ammonia spike, killing all the fishes suddenly.
  • Visible burns on the fishes’ skins/gills/bodies of the fishes, leading to further complications.
  • Restlessness in fishes.
  • Algae thrive in high pH as well. Algae blooms are harmful to the fishes.

How to Lower pH in an Aquarium

Remember to reduce the pH gradually.

  • Mix some distilled/RO water (without re-mineralizing) in the aquarium and find ways to correct the pH of the replacement water/ change the replacement water source for the future.
  • Use driftwood or peat moss in tank decor. Both reduce the pH gradually by releasing tannin.
  • Drop an Indian Almond (catappa) leaf in the fish tank. It will lower the pH in 3-5 days.
  • Remove excessive plants, or fix the filtration system, as the case may be.
  • You can lower the pH in your aquarium with vinegar. Add 1 ml of distilled white vinegar/per gallon of water to the fish tank. Remember, your tank also has substrate and decor, so reduce the gallon capacity by 8 to 10% while calculating.
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How to Balance pH in Fish Tank

Irrespective of whether you have a variety of fishes/single species in your fish tank, it is challenging to maintain a specific pH value, considering that the pH fluctuates multiple times for various reasons. Instead, aim for a pH range that permits fluctuations within itself. This approach will make it easier to balance the pH in the fish tank.

To achieve this, manage the alkalinity of aquarium water. As long as the GH/KH value is in an acceptable range, your aquarium will be able to buffer pH fluctuations and maintain a stable pH, which is the key requirement.

Acclimate the new fishes to your aquarium water (pH). Most fishes accept gradual changes in pH successfully.

Accurate pH is necessary only for breeding. If you plan to spawn fish, use a separate tank for the breeding pair(s), fish eggs/ frys, and very young fishes, as they all have stringent pH requirements.

Below are the pH and KH values for different types of aquariums and the ideal pH for breeding some of the popular aquarium fishes.

Aquarium type Required pH Required KH (in ppm)
Freshwater Aquarium 6.5  – 7.5 70 – 140
Saltwater /Reef Aquarium 8.0 – 8.4 142 – 215
Fish Species Breeding pH Water Hardness mg/L CaCO3
Goldfish 7.0 – 7.5 90 – 200
Koi Carp 7.0 – 7.5 70 – 200
Angel Fish 6.3 – 8.5 70 – 200
Gouramis 6.0 – 7.0 60 – 100

6.5 – 7.5

80 – 250

Note: Betta fish require a pH for 6.5 to 6.8 for breeding.

Parting Thoughts

Interestingly, although most fishes we purchase are captive-bred, their genetic structures still require a pH value of the water similar to their original habitat (although they have never been there) to breed. Hence, some fishes do not breed in captivity.

Hopefully, you are more confident about balancing the pH in your fish tank now. Closely monitor the pH of your aquarium with every water change. You can use Litmus papers, a testing solution, or a digital pH meter. A pH meter gives the most accurate reading.

According to the US Environment Protection Agency (EPA), CO2 in the atmosphere has increased by 40% post-Industrial Revolution in the 21st century. It has spiked Ocean acidity by 25% (from 8.1 to 8.2) for the first time in millions of years.

This has already impacted the aquatic life of creatures having external calcium shells, endangering their existence. Not all can migrate to safer waters. Watch the video to understand how it destroyed oysters.

It is time to minimize our carbon footprints. We wish you a happy fish parenting time!

Recommended Further Reading:

How long can goldfish go without food

About the Author

Victoria Lamb

Victoria is a freshwater aquatics specialist, fish keeper, and amphibian enthusiast. She has had more than 6 years of experience caring for aquariums and keeping several fish species, and her home boasts of 3 aquariums and a garden pond. Her goal is to educate fish owners on raising healthy and happy aquatic pets. Career Highlights:…

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