how to lower ph in aquarium

How to Lower pH in Aquarium: Safe pH Adjustments

Do you need to learn how to lower pH in aquariums? In this how-to lower pH in aquarium guide, you’ll learn how to lower pH in aquariums safely so you don’t stress your fish and aquatic creatures.

It is important to have the correct pH level for your fish tanks. Perhaps more important than the precise, ideal level is a consistent pH level. Fish are adaptable to consistent but slightly imperfect pH. We have included techniques that will help you maintain a consistent pH level so that adjusting the pH is (hopefully) a one-and-done type situation for you. Learn how to lower pH in aquariums.

Recommended Items to Lower pH in Aquarium:

What is pH?

Time for a high school chemistry refresher. pH is the measurement of how acidic or basic a water solution is. But what does that statement mean?

It is important to understand the larger concepts around pH. Water (H2O) is made up of hydrogen and oxygen. Two hydrogen atoms, one oxygen atom.

In water, some of the water molecules get split up and lose one of their hydrogen atoms. Now they only have one hydrogen atom, bonded to the oxygen, and become hydroxide ions (OH-). The free-floating hydrogen ions (H+) are separate now.

In pure water, there are an equal number of free-floating hydrogen ions (H+) and hydroxide ions (OH-). 

However, when a basic (alkaline) or acidic substance dissolves in water, it will alter the balance of ions by attracting, and bonding with, some of the charged ions floating around. Opposites attract so negative bonds with positive, and vice versa. When this occurs, it changes the pH of the water.

An acidic substance will increase the number of hydrogen ions (H+), lowering your pH. 

A basic substance will bond with the hydrogen ions (H+), resulting in more hydroxide ions, and raising the pH level.

What is the pH scale?

Scientists measure pH on a pH range of 0-14 with 7 being neutral. Below 7 is acidic and above 7 is basic (alkaline).

Scientists created the pH scale because each single increase in pH value (going from 6 to 7, for example) represents a 10x increase in concentration. Therefore, you could have a number with 14 zeros (going from 0 to 14) if you’re comparing a strong acidic solution against a strong basic solution. Nobody wants to deal with that much math constantly (Is it a trillion? Is it a zillion?) so they invented the relative scale to describe the range in pH.

Testing ph in aquarium

To translate all of this to aquarium terms, if your aquarium has a pH of 6 it’s 10x more acidic than a tank with a pH of 7. If your aquarium has a pH of 5, it’s 100x more acidic than a tank with a pH of 7. Basically, add a zero for each single step on the pH scale.

What influences the pH of your aquarium?

Lots of different factors influence the pH in your freshwater aquarium. Here are a few examples:

Elements that will lower the pH:

  • Driftwood
  • Adding CO2
  • High nitrates
  • Dirty water filled with waste
  • Overstocked aquariums
  • Soft water has low pH

Elements that will increase the pH:

  • Hard water has high pH levels
  • Crushed coral substrate

As you can see, there are a myriad of different factors that affect aquarium pH. However, pH stability is the most important factor as fish can adapt to a pH that is slightly different from their preference. The biggest source of stress for fish is pH fluctuations. 

What are the effects of pH in an aquarium?

First, stable pH is preferred, as long as it is not too far off from ideal. So if your fish are happy and stable, please avoid trying to get the “perfect” pH just because you want to reach a certain number. However, if your tank is too extreme either direction, here are some effects you can anticipate:

High Alkaline

High alkaline (basic) water can cause non-toxic ammonia to become toxic. Fish will have trouble breathing. If the alkalinity remains high over a prolonged period, it can dissolve the protective mucus layer that covers fish, leaving them more vulnerable to infection. Further, highly basic water will chap their skin, leaving them further exposed to infection. Left for a long period of time, high alkaline water will kill your fish.

High Acid

An acidic aquarium can result in the production of excess mucus by your fish; they’re trying to protect themselves but this excess mucus will have negative effects over time. Namely, it will make it more difficult for them to breathe. Other observable symptoms include gasping, thickening of skin and gills, and eye damage. Highly acidic water will also stop fish eggs from hatching. As with high alkaline environments, fish death can occur.

When is it necessary to lower the pH in your aquarium?

Many aquarium fish and aquatic plants come from areas in the world with stable water quality parameters. These are the water parameters that an aquarist tries to recreate in the fish tank.

For example, tetras and Amazon Swordplants come from tropical regions like South America and Southeast Asia, where slightly acidic water is prevalent. The natural streams, rivers, and lakes where these fish and plants live flow through jungles that drop leaves, sticks, and driftwood. Organic matter has the tendency to acidify the water so it is no surprise the water they inhabit is on the acidic end of the scale.

Although many aquarium fish are captive-bred and less stringent about their pH requirements than their wild peers, for best results, it is still advised to adhere to their natural preferences as closely as possible. If your fish prefers an acidic environment, keeping them in a basic environment long-term will stress them over time, exposing them to disease and shortening their lifespan.

So what do you do if you need to lower the pH in an aquarium?

How to Lower pH in Aquarium?

There are a number of methods to alter the pH level in your tank. Some of them involve chemicals, and some are natural methods. If at all possible, we advocate for the natural methods that mimic your fish’s natural environment. However, we also understand that sometimes, natural methods are not possible so chemicals must be involved.

It is important to remember the golden rule of pH adjustments: don’t make changes too quickly or too drastically. You will stress your fish, and likely kill them.  With all water chemistry parameters, slow changes are better.

Below is an overview of the best methods for lowering the pH of your water.

How to Lower pH in Aquarium: Methods

Chemical Solutions

This is likely the fastest and one of the most common ways to lower the pH in aquariums. However, it does come with risks if done too abruptly, so always be careful when using chemical solutions.

The active ingredients vary with chemical solutions. Some use diluted acids and others using natural methods like tannins and other organic matter with acidic qualities. All should be used specifically according to the directions on the label. 

Additionally, these products should not be used in substitution for regular tank maintenance. It is important to test your aquarium water weekly in addition to a weekly partial water change of no more than 25% to reduce toxic buildup, remove excess debris, and replenish the oxygen available.

Peat Moss

Peat Moss is a natural way to safely lower your aquarium’s pH.

However, it can and will discolor your water. Think brewed tea color. For some situations, like neon tetra breeding, this is ideal. However, most aquarists prefer clear water.

To avoid this, you can pre-treat it in a separate container for a few days before putting it in your tank. This will allow some of the initial color to settle out in the other container.

You can buy pellets or chunks of Peat Moss and place them in your filter. From there, it will release acidic substances into the water which will decrease the concentration of the bicarbonates in your water, effectively lowering the pH.

Volume of peat moss used depends on the starting conditions of  your tank. It’s best to start off with a small amount and monitor your pH from there. 


Adding Driftwood to your aquarium will safely lower its pH levels. Similar to Peat Moss, driftwood will release tannins into your tank’s water, reducing the pH.

Like peat moss, it will also tinge your water yellow/brown. This discoloration isn’t harmful to your fish or plants but it can hurt your aesthetics if you want a crystal clear tank.

Make sure the driftwood you use is marked safe for aquarium use. Driftwood sold for reptiles can contain chemicals which are harmful to your fish. Additionally, be careful about adding naturally harvested driftwood to your tank as it can contain parasites or disease.

Before adding driftwood to your tank, you can boil it in salt water to sterilize it. This will also help stop the Driftwood from coloring your tank water significantly.

Indian Almond Leaves / Catappa Leaves

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SunGrow Mini Catappa Indian Almond Leaves

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Indian Almond leaves will release tannins in your tank as they slowly decompose in the water. They’re also said to have antibacterial properties, which is helpful for fish with fin rot. Fry also thrive with Indian Almond Leaves.

Like driftwood and peat moss, they will turn your aquarium water yellow or brown. If you want to avoid this shift, you can pre-soak the leaves before adding them to the tank to capture some of the initial color change. However, must fish species naturally encounter leaves and other organic material in their natural habitats, so your tank inhabitants might just like the water shift.

If you don’t want the actual leaves in your tank, you can also soak the leaves separately and add the stained water to your tank during a water change.

cO2 Reactors

While not used specifically for lowering the pH, co2 reactors provide a constant stream of carbon dioxide. CO2 is mildly acidic and will help lower pH in aquariums. As long as the reactor is running, it will keep the pH lower and stable.

A side benefit of this approach is that your plants will love you in planted tanks! 

Reverse Osmosis

Reverse Osmosis purifies water by passing it through a semipermeable membrane that only allows water molecules to pass through, filtering out water impurities such as pesticides, arsenic, and heavy metals

This is a very effective, safe, and natural way to deionize your water. It also has the benefit of not staining your water’s color. However, it is not without a literal price. A good Reverse Osmosis unit can cost several hundred dollars. 

You should also expect the reverse osmosis unit to require occasional filter replacements. If you have hard tap water, this is a great option.

Tools to monitor the pH in your aquarium

The first step in understanding whether the pH in your aquarium needs to be adjusted is getting an accurate reading. Here are a few recommended tools to monitor the pH.

pH Water Test Kits

We recommend testing your water quality on a weekly basis. It is the best way to understand the intricacies of your tank.

Keep your pH stable

Consider the current condition of your fish before you start trying to alter the pH levels in your tank. Fish prefer ideal conditions but they also prefer stable conditions, as they’re adaptable little creatures. pH change is hard on fish.

Although the ideal pH for your fish might be a perfectly neutral 7.0, your pH level might clock in at 6.8 or 7.2. Just because your number isn’t absolutely perfect does not mean you should immediately spring into action.

If your fish are happy, it’s probably better to keep the pH stable. Your fish could have adapted to your less than perfect conditions and trying to switch the conditions will cause additional stress.

In other words, it is better to have a pH of 7.2 than a pH which fluctuates between 6.6 and 7, even if the textbooks say your fish prefers 7.

Always remember that your weekly partial water changes, weekly water parameters testing, and routine aquarium cleaning is key to keeping your pH levels stable.

Aquarium pH FAQs 

What happens if pH is too high in fish tank?

If your water is overly alkaline (basic), your fish will have trouble breathing. It can dissolve the protective mucus layer that covers fish, leaving them more vulnerable to infection. Further, highly basic water will chap their skin, leaving them further exposed to infection. Left for a long period of time, high alkaline water will kill your fish.

At what pH do fish die?

Most aquarium fish thrive in tanks with pH that ranges from 6.8 to 7.6. Fish reproduction is negatively affected at a pH of 5. Eggs will fail to hatch at pH 5 or below. Most fish will begin to die when the pH reaches 4.

Can I use vinegar to lower the pH in my fish tank?

Yes, you can use vinegar to lower the pH. However, it is only a temporary solution.  When vinegar is added to your tank, ionization, or the process of releasing hydrogen ions from water molecules, occurs instantly. Then, the acetic acid, combined with oxygen, will convert itself into carbon dioxide. This also results in a decrease in pH.

However, care must  be taken with this approach. You must be gradual in the approach; otherwise, you will stress your fish with a rapid decrease in pH.

The advised dosing of vinegar per gallon of water is 1mL, which will lower the pH about 0.2 – 0.3.

Can I use baking soda to lower pH in fish tank?

Baking soda will not lower the pH in your tank; it will increase the pH! Be careful not to add too much at one time and cause a major change in the pH as this could kill your fish. 

The advised dosing of baking soda is teaspoon per 5 gallons. Dissolve the baking soda in water before adding it to the tank so the chemical reaction occurs outside the tank.

How to lower pH in Aquarium: Conclusion 

Knowing how to lower the pH in aquariums is an important skill in fishkeeping. There are multiple ways of achieving a lower pH and each method has pros and cons.

If you’re making adjustments with fish in a planted aquarium, be careful to follow any directions that come with your additive of choice, go slowly with changes, and regularly test the water to ensure your parameters don’t get out of hand. Your pet fish will appreciate the slow changes.

Have you successfully changed the pH in your tank? What was your experience like?

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