How to Lower pH in Aquarium: A Guide for Tannin-Rich Water and Acid-Loving Fish

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Just because water is clear and sparkling, that doesn’t mean it has to be without color.

Yes, that’s correct.

In fact, bettas and other acid-loving fish prefer water that has a slightly tannish color. 

 “Blackwater” or tannin-rich tanks have a lower or more acidic pH that has several benefits for fish.

Knowing how to lower pH in an aquarium lets you create the best habitat for some of the most beautiful fish in the world.


On average, an excess of positively charged ions will shift the water away from a neutral pH level towards acidic (lower than 7.0).

Aquatic creatures that originate in acidic waters need a lower pH level than neutral to thrive. In particular, when fish are juveniles, they are more sensitive to the pH range in the fish tank.

There are three types of low pH tanks for freshwater fish:

  • First, you can use chemical additives to keep the pH level on the lower side of the spectrum. Usually, these additives do not cause a significant or long-standing change in the water’s color. This holds true if you used bottled water or tap water.
  • Second, you can use lighting effects, plants, and other natural methods to keep the tank water stable at a lower pH. 
  • Third, you can use a combination of driftwood and decomposing leaves to create a “blackwater aquarium”. This tank water is usually the color of lightly to well-steeped tea.

All three types will work for acid-loving fish, however, you may find one more aesthetically pleasing than the others.

Regardless of how you decide to lower the pH in the tank, the water itself should still be very clear and easy to see through.

Usually, healthy tank water has no foul, bitter, or strong odor to it. 

Distilled Water,  Reverse Osmosis Water and pH

hands of an aquarist pouring water in an aquarium

I’ve seen some truly bad answers to the question of how to lower the pH in an aquarium.

More than a few people have killed one tank of freshwater fish after another by insisting on using reverse osmosis water for their tanks without addressing mineral replacement.

Even though reverse osmosis water may have a neutral pH, it also has very few, if any minerals in it. Water that has no minerals in it is deadly to fish, even ones that do best in soft water.

“Pure”, or distilled water isn’t the same thing as healthy water for an aquarium. Fish and other aquatic creatures need the salts and minerals dissolved in water to live.

You are much better off using a chelating agent and tap water chemical neutralizer to prepare water for the aquarium.

It is also much easier to remove excess general and carbonate hardness than add them back to distilled water or water purified by reverse osmosis. 

Pros and Cons of a Low pH Tank


  • Since ammonia is alkaline, acidic water can act as a slight buffer.
  • Acidic environments tend to carry fewer dangerous diseases. 
  • Tannin-rich water can help fish heal faster from injuries.


  • It may be visually difficult to determine if there is an alga bloom in a blackwater tank.
  • A pH that is too low can interfere with gill function.
  • May need stronger lighting so that enough reaches to smaller plants at the bottom of the tank. This, in turn, can increase algae risks.

3 Ways to Create a Tannin-Rich or Blackwater Tank

1. Driftwood and Live Plants Combined

The easiest way to create a tannin-rich tank is to start off with pieces of driftwood. When you buy them, make sure they haven’t already been leached of their tannins.

The driftwood will steadily release acid as it decomposes. This will form the long-term basis for stabilizing the pH at the lower end of the scale.

Next, you will need to use live freshwater aquarium plants that tend to shed a lot of leaves. Aponogetons, Java Moss, and Anubias are all excellent choices. 

2. Using Live Plants to Create Tannins

driftwood in an aquarium

When using live plants to create acidic water, you do not need to trim the plants as much or remove all the dead leaves.

The amount of debris you remove will depend on the pH you are aiming for and the size of the tank.

If you are planning to keep aquatic creatures that like to dig in debris or the substrate, this can lead to a very messy-looking tank.

Monitoring the pH of the tank weekly is also very important when you use decayed leaves from live plants to generate tannins.

Too many leaves decaying can cause the pH to drop too low and also generate ammonia.

If the pH goes too low, nitrifying bacteria may become inactive or die off.

Aside from creating an ammonia surge, the lower pH may also be unsuitable for some plant species in the tank.

The generation of a runaway plant die-off is just as dangerous as leaving dead fish or other creatures in the tank.

3. Using Catappa Leaves

You do not have to choose between a tannin-rich tank and a neat and tidy one. Catappa Almond leaves give you the option to add tannins to the water without unsightly live plant shedding.

No matter whether you place them in a nylon stocking or block them off in some other way, they will still release tannins into the tank without leaving a mess in the process.

Alternatively, you can simply let the leaves float around in the tank. Bubble nest builders, in particular, will make good use of the leaves for anchoring their nests near the surface of the water.

I recommend using Catappa Almond Leaves for beginners and small tanks. They give you an excellent chance to learn about the benefits of a tannin-rich tank as well as how to manage one. 

Lowering and Stabilizing pH Using a Combination of Chemical and Organic Means

As with increasing pH, there are a number of liquid and tablet-based formulas you can use to lower the pH in your tank. All you have to do is follow the instructions on the package and test the water at regular intervals.

Using chemicals to lower the pH will work in an emergency situation. The challenge comes in when you try to stabilize the pH over weeks, months, and years.

Without the usage of organic materials like crushed coral (raises KH), and peat moss, you may wind up with dangerous fluctuations in the pH of your fish tank.

Adding peat moss to the filter will slowly lower the pH and keep it stable. Peat moss will also strip a range of minerals from the water. If the fish love soft, acidic water, this may be a good option for your fish tank.

On the downside, peat moss may darken the water for a few days after adding it to the filter.

The Role of Light on Plants and Algae in pH Fluctuations

fish swim under sunlight

Aside from the natural life cycle of growth and decay, there is another way live plants and algae affect the pH in a fish tank.

During the day, or when there is enough light, plants and algae absorb both carbon dioxide and nitrates. Both of these serve to acidify the water.

Therefore, when plants absorb nitrates and carbon dioxide, the pH goes up.

When there isn’t enough light in the fish tank; nitrates and carbon dioxide increase, thus lowering the pH.

I don’t personally recommend leaving the aquarium in the dark as a means to lower the pH. This is the fastest way to suffocate fish, especially if there is an alga bloom in the fish tank.

 I do, however, recommend keeping the fish tank out of direct sunlight and using less overall lighting in the aquarium.

Less lighting will help keep algae growth at a lower level, and also help control live plant growth.

Just make sure that there is still enough light to help ensure the plants make enough oxygen during the day.

How to Manage Low pH During an Ammonia Surge

As long as the pH is still above 6.0, nitrifying bacteria will continue to process ammonia. Therefore, you don’t need to adjust the pH upward in this situation.

An ammonia surge in an established fish tank is still an indicator that something is seriously out of balance.

You can, and should use zeolites and other ammonia absorbers to neutralize the ammonia as quickly as possible.

If the pH is approaching 6.0 or at this level, you need to raise it to around 6.5 – 6.8. You will need to do this slowly so that you do not shock the fish or other aquarium inhabitants with excessive fluctuation.

After you get rid of the ammonia and bring the pH up, you may need to introduce more nitrifying bacteria into the fish tank.

I recommend keeping a small freshwater aquarium or fishbowl ticking over with live plants and some snails as a backup system for your biofilter.

Unlike commercial nitrifying bacteria starters, the water from your backup tank will be up and running a good bit faster.

Frequently Asked Questions

When is it necessary to lower the pH in aquariums?

There are 3 times when you need to lower the aquarium pH:

  •  When you cycle the tank for the first time in preparation for fish that require a pH below 7.0.
  • When the pH is at 9.0, or around 8.8, and trending up. At 9.0 ammonium converts back to ammonia, which can generate a deadly surge. 

This type of high pH problem is more likely in older tanks than new once, since ammonium builds up over time.

  • When the tank water pH goes too high for the fish and other aquatic creatures living in the tank. 

Will plants lower the pH in aquariums?

It is more accurate to say plant debris steadily lowers the pH in the aquarium water.

Live plants, however, cause the pH to drop only during times when they are actively processing carbon dioxide and nitrates.

At night, or when there isn’t enough light, pH raises again because the plants aren’t engaged in photosynthesis.

Does vinegar work to lower the pH in aquariums?

You can lower the pH in an aquarium using distilled white vinegar, however, it is not the best solution.

To use vinegar safely, it is best to remove some of the water and then reintroduce it a little at a time. Most edible kinds of vinegar have a pH of around 2.0 to 2.5.

If the vinegar isn’t well diluted, it can easily cause acid burns on any fish that swim through it.

Similar to baking soda, the acetic acid in vinegar releases carbon dioxide. While this does serve to lower the pH it can also suffocate the fish in the tank.

You are better off using a commercial formula for lowering the pH or adjusting the ecosystem in the fish tank to keep the pH stable.

In a situation where you have to choose between vinegar and a water change, I recommend trying a partial water change first.

Now, before we get this into a wrap, let’s watch this short video to learn how to lower the pH level in your aquarium, naturally:


Once your freshwater aquarium is established, lowering the pH can seem like a constant need. This is why it is very important to learn how to stabilize the pH at a lower level using the natural ecosystem of the fish tank.

As a last resort, you should still keep pH-reducing chemicals on hand to manage an emergency. You may still need to do a good bit more to manage other problems that come up.

Last Updated: February 11, 2022

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