Proper pH For Freshwater Aquarium

No matter if you own an Aquarium or operate a swimming pool pH is an important function in a chemical or biological action that is the basis for reactions that take place in both worlds where the basic knowledge is needed. One critical method for Freshwater fish Aquariums is the Nitrogen Cycle where pH is the absolute control that can make or break your fish’s environment. What is the Proper pH of a Freshwater Aquarium?

Most well run Freshwater Aquariums contain:

Nitrosomonas and Nitrobacter bacteria that prefer 7.0 to 8.0 pH
Living Plants thrive at 6.5-7.2
Most Freshwater Aquarium Fish with the exception of a few prefer a higher alkaline or low acidic range of pH
Overall Aquariums run best at about 6.8 to 7.6 pH 


Raising tropical freshwater fish is a great hobby for anyone who can see the beauty that can be maintained on a small level and contained in a small space. The same requirements that are necessary for a Freshwater natural fish habitat are basically the same requirements needed for a Freshwater Fish Aquarium. This includes the most important control of pH.


Proper pH for Freshwater Aquarium


In science one method used to describe chemicals is pH or “potential of hydrogen”  pH is a measure of hydrogen ion concentration, a measure of the acidity or alkalinity of a solution. The pH scale usually ranges from 0 to 14. Aqueous solutions at 25°C with a pH of less than 7 are acidic, while those with a pH greater than 7 are basic or alkaline.

The use of pH is an important part of many applications and processes. It is a control used in wastewater treatment, agriculture,  industrial processes, environmental monitoring, and research and development. It is important in Chemistry and Biology to know the acidity or alkalinity of a solution.

The pH value states the relative quantity of hydrogen ions (H+) contained in a solution. An equal amount of hydrogen ions (H+) and hydroxide ions (OH-), is said to be neutral (pH 7.0).  The greater the concentration of H+ the more acidic the solution and the lower the pH. In this relationship, pH is defined as the negative logarithm of hydrogen activity. A standard pH measuring system consists of three elements: 1) pH electrode; 2) temperature compensation element and 3) pH meter or controller.

Ph factors are increased and decreased by factors of 10.

So a pH of 6 is 10 times more acidic than a pH of 7.0 or a pH of 5.0 pH is 100 times more acidic than 7.0 pH and so on. Measuring pH in water used for tropical fish or any life-sustaining process where standards practices for quality controls are used.

  • This should be done in a consistent matter
  • The same amount of sample
  • Near the same temperature as possible.

You don’t need to understand the principles of pH and how it chemically works in the world. You only need to understand the basic principles that make it a control test and how it’s important that it stays in a certain range for the quality of the water and the health of the fish you are keeping in the tank.

If you know where it is, and how to keep it in range, you can prevent problems that will happen before they start to affect the health of your fish.

The measure of whether water is acidic (pH 1 to 7.0) or basic (pH 7.1 to 14). 7.0 is considered neutral. Most freshwater aquarium tropical fish do best at a pH of 6.8 to 7.6, although certain fish may require higher or lower levels. The pH of an aquarium tends to drop over time due to the breakdown of organic material, and the best way to prevent this is through regular maintenance and partial water changes.

Symptoms of Low pH in Fish Aquarium


Most freshwater fish tanks will run very well at pH levels of around 6.8-7.6 but there are breeds of fish that prefer the pH at different ranges than other freshwater fish. The object is to find that pH and consistently keep the tank around the same level without letting the level slip and change the chemistry of the environment. Natural Biological elements and processes that make up a healthy aquarium needed to sustain life are pH-dependent and will fluctuate during these times.

One of the biggest life-sustaining Biological processes that happen in the Aquarium is the Nitrogen Cycle.

This will happen when fish waste and excess food are broken down into either ionized ammonium (NH4), or un-ionized ammonia (NH3). Ammonium is actually not harmful to fish, whereas, ammonia is toxic. The pH of the water in the aquarium plays a huge role in the health of the tank and the fish.

If the pH is slightly lower in the tank then higher concentrations of ammonium (NH4) are produced. If higher pH levels are found in the tank then that is directly proportionate to a higher amount of Ammonia. If the pH level increases above 9, most of the ammonium in the water is converted into toxic ammonia (NH3), which can kill fish.



The Nitrosomonas bacterium feeds on ammonia and converts down-converting it into nitrite (NO2). Nitrite is slightly less harmful to fish than ammonia, but it is still not very good to have around in the tank. The Nitrites use oxygen and too many can be detrimental to the fish living in the aquarium tank. There is another bacterium that has a role in the process called a Nitrobactor that feeds on Nitrites in the water and converts them to Nitrates (NO3).

Fish can live and thrive with higher levels of Nitrate than high levels of Ammonia and Nitrite. Once the levels of Nitrogen are converted and broken down in the cycle, to Nitrates with this type of bacteria (Nitrobacter) then a sufficient population is built up and the aquarium water is said to be “Cycled” and a safe environment for fish. Plants are very helpful in the nitrate phase of the nitrogen cycle because they feed on nitrates (NO3) Plants will thrive at a pH of 6.5-7.5.

The proper pH for freshwater aquarium water needs to be in a certain range for this all to take place. If the pH drops below 6.0 then the whole process will stop. If the pH rises above 9.0 then very high concentrations of toxic ammonia (NH3) are present. The pH will naturally drop during this whole cycle and may need to be adjusted.

So you can see the whole system is a balance dependent on pH. The bacteria that break down nitrogen along with plants provide a natural habitat on a smaller scale than will happen in nature outside in a much bigger one.

Adjusting pH in Fish Tank Freshwater Aquarium


The pH of the water in your aquarium will move up or down depending on what’s in your tank. Rocks, supplies, and aquarium plants that are stationary in the tank all leech chemicals that will affect the pH balance of the water. Before dressing up your tank with anything you need to establish a baseline for pH.

Once you know this base, then if any fish or items you place in your tank move the pH the items you enter into this environment will be obvious to you as a natural way of lowering or raising the pH without adding chemicals.

Even different sources of water will have different levels of pH. Tap water and filtered water carry different pH. If you have a Reverse Osmosis water filter then you will see a lower pH than a Tap water source that will normally carry higher levels of alkalinity and pH which is added at the treatment plant before it’s sent to your home.


If you want to measure these water sources let the water stand for a day or 2 under aeration and you’ll get a fairly accurate readout of pH. This is done before anything is added to the aquarium except water. Then when you know what the pH of your source water will be you can use it as a natural way of pH adjustment.

Tap water under aeration will separate carbon dioxide gas from the solution of the water and add it into the air exchanging carbon dioxide for oxygen.  This will naturally raise the pH of your source water in the tank. You can also lower the pH by decreasing the aeration time in your tank.

You can use this natural process to raise the pH in your system. Some species of aquarium freshwater fish thrive at certain pH levels either higher or lower levels for you to keep your tank levels at. This knowledge is something for you to be aware of before buying and adding fish to your aquarium.

Natural items like dolomite chippings do the job over periods of time normally a few weeks and are a better approach to keeping it where it needs to be. Crushed Coral is one product that can raise the pH of your tank after a few weeks of being in the water. Another is Driftwood which looks great in your tank and can add a brown-colored chemical called tannins that are leeched from the wood into the tank. All these natural pH adjustments along with Chemical Up and down pH can be found wherever you buy your Aquarium products and accessories.


Tanin has acidic natural properties that will lower the aquarium’s pH water levels. Certain types of freshwater fish find a lower acidic pH as preferable to live and thrive in. Before you purchase fish for your Aquarium get that information you need to keep groups of fish together that prefer the same types of pH.

The problem with Tanin is that the leeched brown-colored chemical that holds the naturally low pH dissipates after a while when the water runs through filtering cycles. You would have to replace it to keep the pH low naturally.

Peat Moss does the same thing. You can add a bag and deliver it like a tea bag through the filter in the tank. Tannin has also added the same way in a brown color that will eventually dissipate

Baking Soda is an alkaline product that is about 8.0 on the pH scale. This is a fast-acting way to raise the pH in your fish tank. Remove your fish then Add a teaspoon full per 5 gallons testing along the way only then reintroduce your fish back to the aquarium after you raised it to a proper level.

Over time, the pH levels in the aquarium will drop in small increments. The best way to counteract this phenomenon is by doing regular water changes. The water changes should be once per month and 25% of water changed.  This will help keep the pH more consistent over long periods of time. Using constant aeration in the tank can slightly elevate the pH levels.


Freshwater Fish Aquarium pH Controller


Altering the pH of the aquarium while the fish are in the water can have risky consequences too much of a change can be deadly. Remember that everything is done in small incremental changes especially when fish are involved.

The pH scale is logarithmic, meaning each whole number change results in a factor of 10. An Aquarium pH Controller can keep your Aquarium and the fish that are living there happy by keeping the pH at a perfect level.

This is a huge change going from 7 to 8.0 in pH which is too high. Researching what pH is needed for what fish is extremely important. Tap water as we looked at before can have a wide range of pH. Saltwater fish can be happy and thrive at 8.0 pH. The majority of Freshwater fish are healthy and thrive at a pH of 5.5 to 7.2.

The main exception to the freshwater range is African cichlids. These fish like harder water (water that is high in mineral content) with high pH. African cichlids from Lake Malawi like a pH of 7.7 to 8.6, whereas African cichlids from Lake Tanganyika prefer a pH of 7.3 to 8.0. South American cichlids, on the other hand, do best in an appropriate pH of 6.5 to 7.2

Other fish like the Discus are very sensitive to elevated pH levels in aquariums. These types of fish need soft acidic water of 5.0 to 6.5 pH.  Plecostomus species can tolerate a fairly wide range of pH, but they like it on the acidic side, from 5.0 to 7.0. Angelfish do well with an ideal pH of 6.5 to 7.0, and goldfish like it a bit higher, from 7.0 to 7.5. These are just a few examples of how bio-diverse aquatic life can be in the aquarium, and this is just a single water parameter.


Buying a reliable test kit like this one found through MyWaterEarth&Sky on Amazon Seneye Home Aquarium Monitor can keep your system properly monitored as this will pay off. Know the difference between pH and Alkalinity which is not fully understood in all water-based hobbies like aquaponic, swimming pools, and Fish Aquariums. The water KH, or carbonate hardness, is the water’s ability to absorb and neutralize the acid. KH is also referred to as alkalinity, not to be confused with alkaline on the pH scale.

Beneficial bacteria like Nitrosonos and Nitrobacter bacteria prefer 7.0 to 8.0 which would satisfy most fish. A good range for fish, plants, and nitrifying bacteria would be 7.0 to 7.2 pH to keep kh in check in aquarium water.

Some prefer a slightly higher range, from 7.0 to 8.0. In general, a good pH range that would satisfy most fish, along with living plants and bacteria would be 7.0 to 7.2. It is important to note that pH in the aquarium is not completely stable, with small fluctuations occurring throughout the day and night. Many things can affect the stability of the pH within a system. Aeration, gravel,  tank water, decorations temperature, and nitrate levels are just some of these.

Remember that water quality has a direct impact on the health of your fish, it’s important for aquarium owners to understand basic water chemistry in order to adjust it correctly. Aquarium owners who learn the basics of water chemistry find it much easier to maintain and enjoy a healthy and safe environment for aquarium fish.



 JimGalloway Author/Editor



Pet CentralThe Role of pH In The Aquarium Nitrogen Cycle-

Lab DepotDefinition of ph 


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