How To Lower The pH In Your Freshwater Aquarium

by Aquarium Scoop | Last Updated: October 26, 2022

Fish are sensitive to subtle changes in pH. Lowering the pH in your aquarium will depend mostly on the types of fish you keep. This guide will get you started.

In this guide you’ll learn:

  • What is pH?
  • What is the correct pH for your aquarium?
  • What causes pH to change?
  • How to maintain consistent pH

Getting the correct water quality and maintaining it is one of the skills you have to master in aquarium keeping. The pH of the water in your aquarium is one of the things you’ll have to watch out for since fish, other aquatic animals, and plants can be sensitive to subtle changes in the pH level. What’s more, the pH level in your tank also affects the water chemistry (e.g. water with higher pH levels can have more ammonia which is toxic to fish). Keeping the pH in your aquarium consistent will avoid unnecessary stress for your fish and other aquarium inhabitants.

What is pH?

The pH level of a substance is a measure of how acidic or alkaline (basic) that substance is. The pH scale is in negative logarithmic scale (based on 10) and is from 0 (most acidic) to 14 (most alkaline) with 7 being the neutral point. Water is usually neutral and has a pH of 7. 

pH scale

The “p” in the term means “power of” since it is in a mathematical negative logarithmic scale and the “H” stands for hydrogen ions (H+). This just means that for every increase in the pH scale, the “acidity” or “alkalinity” really increases tenfold. 

The pH is a measure of the concentration of hydrogen (H+) ions and hydroxyl (OH) ions in a solution which are usually in a balance or equilibrium with one another. If this balance is tipped towards hydrogen (H+) ions, the solution is acidic and if more hydroxyl (OH) ions form, the solution is alkaline (basic). Equal levels of both ions means the substance is neutral at pH 7. 

There are many methods of measuring pH differing in ease of use and accuracy. There are pH probes that you dip in a solution to measure its pH and easy to use color changing strips that you can match against a color chart to estimate the pH.

Expert Tip

It’s best to be familiar with the pH requirements of the freshwater fish you want to keep in your aquarium before getting one so you know how to maintain it accordingly for their health. 

Can I Use Vinegar To Lower pH?

Yes, adding vinegar to water lowers its pH. Vinegar usually found in the kitchen (distilled white vinegar) contains acetic acid and has a pH of 2-3 making it an acidic substance. 

Use vinegar cautiously and with constant monitoring of the pH levels. Too much vinegar can harm fishes. It’s also recommended that you have a substantial amount of plants in your tank before doing so to offset big changes in pH by absorbing excess carbon dioxide. The amount of vinegar needed is only as little as 1 ml per 1 gallon of water. 

Vinegar is usually used as a quick solution to lower your aquarium’s pH since these can also contain other impurities that may be harmful to fishes.

The Proper pH of a Freshwater Aquarium

In aquarium keeping, you need to maintain a stable pH that is suited for the type of fish and plants you have. This level is usually a case-to-case basis depending on the fish but for most freshwater, tropical aquarium fish, the proper pH level is between 5.5 to 7.5 which is slightly acidic to neutral. For saltwater fishes, the proper pH level is a bit more alkaline at about 8. 

Graphic card describing the recommended pH level for freshwater aquariums, which is 5.5 to 7.5 depending on your fish species

Regular pH maintenance and testing

Monitoring pH and water quality levels is an important part of aquarium keeping. Keeping a record of the water quality levels (including pH) in your aquarium will allow you to check for any subtle changes and act accordingly. Checking at least every two weeks is recommended. 

There are a lot of affordable pH and water quality testers in the market available for this purpose. Proper maintenance and care of your aquarium will also help stabilize pH levels and water quality helping keep your fish and other aquarium animals/plants happy and healthy.

What Causes The pH to Rise In Freshwater Aquariums?

A lot of elements in aquariums can raise the pH levels and make the water more alkaline. These can be plants, substrates, ineffective filters, fish waste, and etc. 

Carbon dioxide uptake by plants

Plants naturally take up dissolved carbon dioxide (CO2) as they photosynthesize to make their own food. Less carbon dioxide dissolved in the water raises pH levels, increasing its alkalinity. Dissolved carbon dioxide in water makes a weak acidic solution similar to tonic water or other carbonated drinks.

If you have an overgrowth of plants and algae (or slime) on the surfaces of your aquarium then these may be throwing off your pH balance by absorbing too much CO2 in the water. Algae overgrowth is also a sign of high pH. A rise in pH levels is not healthy for most freshwater fishes. 

Excess aeration 

Too much aeration of your aquarium water could throw off the balance between the levels of dissolved carbon dioxide and oxygen (O2) by allowing carbon dioxide to escape or bubble out. Less dissolved carbon dioxide in the water can cause pH levels to go up, making the water more alkaline.

Impurities in tap water 

Tap water can have some impurities (minerals and metals) that are alkaline making its pH level higher. This is one of the reasons why tap water should be conditioned first to level out the pH and make it more suitable for aquarium use. 

Using “hard water”

Tap water in certain areas can be “hard” meaning they have a lot of naturally occurring carbonate or alkaline substances dissolved in them. They usually have a pH of 7.5 to 8.0. These are usually because the natural springs where the tap water may come from can have a lot of limestone or calcium carbonate.

Hard water has a lot of buffering capacity because of their carbonate content. The minerals in hard water help resist or “buffer” any changes in pH. Usually, the calcium carbonate (which has the bicarbonate ion) helps neutralize any additional acids resisting changes in its pH.

It’s good to test and monitor the pH of your tap water if you intend to use this in your aquarium. You can do so by keeping tap water in a bucket, aerating it for at least 24 hours, and taking pH readings every hour after. Once the readings are stable, you have a good measure of the usual pH of your tap water.

Otherwise, distilled water, reverse osmosis treated water, and rain water (which is usually acidic) can serve as alternatives.

Type of substrate used

Some substrates can raise the pH levels in your aquarium. Typically this is because of the dissolution of the calcium carbonate (CaCO3) content in certain types of rocks and gravel which releases bicarbonate ions (HCO3) in the water. Bicarbonate ions tend to form a buffer system in the water that throws off the pH levels. Corals and limestones are some examples of substrate with high CaCO3 content.

Before adding the substrate or any other elements to your aquarium, make sure you clean these thoroughly by boiling them in water so that they don’t affect your water quality. A good technique is to soak them in treated water for at least 24 hours before placing them in your aquarium. 

Why Do I Need To Lower The pH In My Aquarium?

The water quality of your aquarium depends a lot on the balance (equilibrium) of different chemical reactions. You essentially recreate a balanced ecosystem in your tank between the fishes, plants, substrate, water, chemicals in the water, and even the air. 

The equilibrium of these chemical reactions are affected by a lot of factors including pH levels. For example, higher pH levels can push the equilibrium of ammonia (NH3) and ammonium (NH4+) towards the more toxic form (which is ammonia, NH3). This will affect the health of your fishes. This means you will need to lower the pH in your aquarium if it gets too high.

Conversely, you also do not want your pH levels to get too low since it will affect the helpful bacteria in your system (nitrifying bacteria). These bacteria help keep the by-products of decomposition (ammonia and nitrites) low, helping maintain a good balance in the water quality. At pH levels below 6.0, these bacteria may begin to die out, allowing ammonia and nitrites to build up which is toxic to fishes at high levels.

Aside from affecting water chemistry, pH also affects some basic body functions of fishes such as reproduction, growth, and the ability to fight off disease and stress. 

Ways To Lower The pH In Your Freshwater Aquarium

1. Carbonating your water

Increasing the dissolved carbon dioxide in your water by pumping in CO2 can lower your tank’s pH levels. 

2. Using peat moss as a filter 

Using peat moss as a filter helps absorb compounds in your water (such as bicarbonates) that make it alkaline. Peat moss is the generic name for moss from the genus Sphagnum. Moss absorbs substances through a process called chelation by releasing chelating substances such as tannic acid in the water. Tannic acid binds with alkaline compounds in aquarium water to help keep a low pH. It does tend to discolor the water making it look like tea but this shouldn’t be a problem for fishes.

3. Adding driftwood 

Driftwood also releases tannins into the water that helps lower the pH as well as adding a nice aesthetic to your aquarium. Darker driftwood will have more tannins.

Expert tip: It is good practice to boil anything you add to your aquarium to make sure they are free from any harmful substances. This will also release some excess tannic acid preventing too much water discoloration in your aquarium. 

4. Adding leaf litter (almond or catappa leaves)

Leaves such as almond and catappa also release tannins as they decompose helping lower the pH of your water. They do so more gradually and discolor the water less. Almond leaves are also reputed to have some antibacterial properties which help in maintaining water quality. 

5. Using chemicals

Common pet stores sell chemicals that you can add to the water to lower its pH. These are usually inorganic salts that dissolved into weak acids when added into water. Chemicals are usually quick fixes that do not keep your pH stable. It’s best to check what is causing the offset in pH by checking the other factors and address those instead.

You can also use alternatives found in your house such as white vinegar to lower the pH. However, use vinegar cautiously, as a last resort, and with constant monitoring of the pH levels. Too much vinegar can harm fishes. It’s also recommended that you have a substantial amount of plants in your tank before doing so to offset big changes in pH by absorbing excess carbon dioxide. The amount of vinegar needed is only as little as 1 ml per 1 gallon of water.

How To Keep pH Stabilized

  1. Proper maintenance and monitoring
    Nothing will beat proper aquarium maintenance and diligent monitoring of water quality to keep everything in check and keep a stable pH.

  2. Keeping your aquarium clean
    Regularly cleaning out or vacuuming waste and uneaten food is the best way to prevent any build up of waste that can cause imbalances in pH. Do a good, thorough cleaning to keep ammonia and other waste levels in check.

  3. Checking your filter
    Check and replace your filter regularly to make sure that it is working well in keeping the waste level in your aquarium low. The filter also helps keep the balance of good microorganisms in your tank that absorb harmful chemicals such as ammonia and nitrites.

  4. Changing your water properly
    Tank water changes should be done properly. Doing this gradually and a little bit at a time helps keep your fishes from getting shocked by the new water addition which may have a different water quality than the one currently in their tank. This is especially true for pH and chlorine levels if you use tap water.

    Keep an extra bucket or two where you can place tap water that you can treat, aerate, and monitor before using in your tank. 

  5. Proper monitoring
    Investing in a good water quality testing kit to monitor the water pH levels and water chemistry of your aquarium is a must. Make sure your testing kit is up to date (they expire) and accurate. 

More Tips For Maintaining pH Balance

Add new objects in your tank slowly

An aquarium is in a very delicate balance wherein any new addition or slight change can cause a big offset. New objects you intend to add such as plants and driftwood (or even fish) should be cleaned and acclimatized first before placing into your tank. 

Keep an extra bucket to “soak” these objects in with an aerator so any harmful chemicals can leach out into the water before you add them into your tank. 

Adding plants, driftwood, peat moss, or leaf litter

Aquatic plants are a great way to help stabilize your tank’s pH and water quality levels. These help maintain a good balance in the water chemistry as they absorb some harmful chemicals and release helpful ones in the water. 

Addition of objects that release tannins in the water such as driftwood, peat moss, and almond or catappa leaves also help keep the pH in check by helping offset any increase in pH.

Invest in a reverse osmosis filter (RO) machine

A reverse osmosis (RO) machine can help stabilize the pH levels of your water. It carefully filters out your water and uses reverse osmosis to get rid of contaminants and even metals. This is quite an investment but serious fish tank enthusiasts swear by its usefulness.


What causes high pH in an aquarium?

High pH can be caused by carbon dioxide uptake by plants (low levels of carbon dioxide raise pH levels), excess aeration which bubbles out carbon dioxide, and impurities in tap water which can have alkaline compounds. 

How can I lower the pH in my aquarium quickly?

Using chemicals found in pet stores (“pH down” products) or vinegar can quickly lower the pH in your aquarium but use these gradually, with caution and proper monitoring as the sudden change in pH can be bad for your fishes.

Can vinegar lower pH?

Yes, common household vinegar (distilled white vinegar) is acidic with a pH between 2-3. Use vinegar cautiously and with constant monitoring of the pH levels since too much vinegar can harm fishes. It’s also recommended that you have a substantial amount of plants in your tank before doing so to offset big changes in pH by absorbing excess carbon dioxide. The amount of vinegar needed is only as little as 1 ml per 1 gallon of water. 

How do I balance the pH in my fish tank?

Keeping a balanced pH in your fish tank is done by proper maintenance and monitoring. Keeping a good cleaning schedule will help keep waste products that throw off pH levels to a minimum. Adding plants, driftwood, and peat moss can also help stabilize and balance the pH levels of your fish tank. 

Does baking soda lower pH?

No. Baking soda is a substance called sodium bicarbonate (NaHCO3). It contains the bicarbonate ion which is a slightly alkaline substance that increases pH levels.

Does lemon water balance your pH?

Lemon water will lower the pH of your aquarium since this is an acidic substance. However, it is not a recommended remedy. It only lowers the pH for a while and it may cause unwanted side effects for your fish. It can also foul your water up quickly.


Some ways to lower the pH of aquarium water include: carbonation, using peat moss, driftwood, and leaf litter, or using chemicals from pet stores. Vinegar should be used sparingly and carefully and should be a last resort. Keeping a stable pH in your aquarium water is done best by practicing good maintenance and water quality monitoring. 

Further Reading

Short but very informative articles on the basics of pH and how they relate to aquatic organisms:
USGS Special Topics: pH and water
pH (in freshwater) – Water Rangers
pH of Water – Environmental Measurement Systems
Effects of acids and alkalis on aquatic life

Informative article on maintaining the pH in a freshwater aquarium:
Properly Maintaining the pH in a Freshwater Aquarium

Some forums on pH levels of aquariums:
Cycling my first aquarium. pH and kH are very high : Aquariums
Aquarium pH too high at 7.5, what could cause it?