How To Lower Alkalinity In Fish Tank Water

While high alkalinity is generally considered ideal for fish tanks, there are times when it may be necessary to lower the alkalinity. Understanding how to treat high alkalinity in fish tank water is a good tool to have in your arsenal.

Below are the most commonly used methods used to fix high alkalinity in fish tank water.

There are several natural methods to lower alkalinity in a fish tank, such as:

  • Regular water changes
  • Adding live plants
  • Using peat moss
  • Adding driftwood
How To Lower Alkalinity In Fish Tan...
How To Lower Alkalinity In Fish Tank Water

And some less natural methods such as:

  • Using a reverse osmosis filter
  • Using a chemical filter
  • Add white vinegar or lemon juice
  • Add distilled water

If your freshwater aquarium alkalinity is too high, you will need to take some measures to reduce it. Whichever way you choose to lower the alkalinity in your fish tank, you will need to make sure the changes happen slowly, as most aquarium fish are sensitive to sudden changes in water chemistry.

How To Lower Alkalinity In Fish Tank

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Water chemistry can be a complicated topic and is often made more complicated than necessary when concerning aquariums.

This article will explain everything you need to know about how to fix high alkalinity in a fish tank, by covering the basics, such as:

  • What Is Alkalinity In Water
  • Alkalinity VS pH In Aquarium
  • What Should The Alkalinity Be In A Fish Tank
  • How To Lower Alkalinity In Fish Tank
  • Is High Alkalinity Bad For Fish
  • Will High Alkalinity Kill Fish
  • What Causes High Alkalinity In A Fish Tank
  • Fish That Like High Alkalinity

What Is Alkalinity In Water

Alkalinity is a measure of the water’s ability to neutralize acids. The higher the alkalinity, the more resistant the water is to changes in pH.

Water with high alkalinity is usually considered “hard” water and contains high levels of dissolved minerals such as calcium, magnesium, and heavy metals.

Minerals dissolved in the water will act as buffers by absorbing acids and helping to stabilize the pH of the aquarium’s water.

Fish tank water with high alkaline levels is much more stable than acidic water, prone to swings in the aquarium’s pH level.

Alkalinity VS pH In Aquarium

Alkalinity is not the same as pH, although they are often confused. pH is a measure of the water’s acidity or basicity (alkalinity), while alkalinity is a measure of the water’s ability to neutralize acids.

When talking about the buffering capacity of alkaline, it is also referred to as KH or Carbonate Hardness which is also different from GH or General Hardness. Carbonate Hardness refers to the level of carbonates and bicarbonates in the water and is measured in ppm (parts per million) or dKH (degree of carbonate hardness).

The pH scale is a measurement system from 0-14, with 0 being acid, 7 neutral, and 14 basic (alkaline). A pH value above 7 is considered alkaline. This alkalinity level is multiplied by 10 for each additional value, so a pH of 9 is 10 times more alkaline than an 8.

Water pH Scale Chart
Fish Tank pH Levels Chart

Understanding that the level of alkalinity increases quite dramatically the further you move along the scale will also help explain why aquarium water with slightly high alkalinity is not so severe.

Your main aim is to find out what pH is suitable for your fish, create that pH in your aquarium and then maintain a stable pH level.

You can understand aquarium pH levels and learn how to lower pH in an aquarium by reading this article: Fish Tank pH Levels (An easy guide for fishkeepers).

What Should The Alkalinity Be In A Fish Tank

The alkalinity in a fish tank is important because it helps to regulate the aquarium’s pH level. The ideal alkalinity range for a fish tank is between 4-8 dKH or 70 and 140 ppm. This range helps ensure that the water is not too acidic or too basic, which can be harmful to fish.

You can measure alkalinity levels with a test kit, and you should check them regularly to ensure that they remain within the ideal range.

If the levels start to get too low, it is often because the fish produce too much waste. It is essential to increase the frequency of water changes in this case.

Finding the best level of alkalinity in a freshwater aquarium will help to create a healthy environment for your fish.

How To Test Alkalinity In A Fish Tank

Testing alkalinity in a fish tank will require the use of a test kit and a water sample. Many test kits are available, and many will test all water parameters at once.

Testing aquarium water is a simple case of dipping a test strip into the water sample and waiting for the strip to change color. You will compare the color changes to a chart that has been provided. Each slight variation of color tone will indicate a different value.

Digital testers are the easiest and most accurate to use, but they only test specific parameters, so you will need to purchase a tester for each parameter that needs testing.

The most common test kits are freshwater test strips, which will test for high ammonia levels in a fish tank, and will test for the presence of nitrites and nitrates. They will also help you to keep a close eye on the aquarium’s pH level and water hardness.

Tetra EasyStrips 6 in 1 Freshwater And Saltwater x 100 Test Strips

Tetra EasyStrips Aquarium Water Parameter Test Kit
Tetra EasyStrips

Tetra EasyStrips are simple to use and will measure 6 of the most common water parameters in both freshwater and saltwater aquariums.

Simply dip a test strip into the aquarium water and compare the color change to the included color chart for an accurate result.

This test kit of 100 strips measures chlorine, nitrate, nitrite, general hardness, alkalinity, and pH.

How To Lower Alkalinity In Fish Tank

There are several methods to reduce alkalinity in an aquarium, some of which are more natural and, therefore, gentler. Natural methods will have a more prolonged effect on the pH.

If you need to reduce pH quite drastically and therefore quickly, there are other methods that use chemical additives or special filters. These will only have a temporary effect, so you will still need to identify why the alkalinity levels keep rising.

All the methods below are suitable for lowering alkalinity and pH. If you need to make drastic changes to the water’s hardness, you will need to remove your fish as they will not tolerate changes faster than 1-1.5 pH per 12-24 hour period.

You will keep them in a holding tank with the original high alkaline water when removing fish. You can then gradually mix some of the low alkalinity water into the tank until they have become accustomed to it, then add them back into the main tank.

Regular Water Changes

Frequent water changes using a low mineral, soft water source such as rainwater, distilled water, or “RO water” (reverse osmosis water) will help dilute aquarium water and reduce alkalinity which you can read more about here: How to soften aquarium water.

A build-up of fish waste and other organic matter will also affect alkalinity as it begins to decompose, so some gravel vacuuming should accompany water changes when providing routine maintenance.

It is best to do small water changes (10-15%) every week rather than large ones as this will help to reduce stress on the fish. Sudden changes to water chemistry will cause a shock reaction in fish which can sometimes be quite severe but, in any case, may lead to further illness.

Adding Live Plants

Live plants help lower alkalinity by consuming carbon dioxide and releasing oxygen into the water. If you want to know how to oxygenate a fish tank naturally, live plants are definitely the way to go. Live plants also help keep the water clean by absorbing nutrients from the water.

Adding live plants is a great way to naturally lower alkalinity levels in a fish tank. They have many other benefits for your aquarium and the biological processes that occur.

If you have lots of live plants, you will need to clean them regularly to remove rotting leaves and food waste that can build and increase ammonia. You can find out how to clean aquarium plants here: How To Clean Aquarium Plants (Best methods revealed).

Using Peat Moss

Peat moss is an acidic material that you can use to lower the pH and alkalinity of water. It is best to use peat moss aged for at least six months, as this will help minimize the chance of it raising the pH levels.

Peat moss can be added to the filter or a mesh bag and placed in the aquarium. As the water passes over the peat moss, it will start to reduce alkalinity.

Adding Driftwood

Driftwood releases tannins into the water, which can help to lower the pH and alkalinity.

It is essential to use natural driftwood that is untreated and therefore suitable for your aquarium as some types can release harmful chemicals into the water.

Pieces of driftwood can be added to the filter or placed directly into the aquarium and used as a feature. Over time, driftwood will release natural tannins into the water, similar to peat moss, and these tannins will help reduce alkalinity.

In aquascaping, many people add plants to their driftwood, which is twice as effective as you will be using both driftwood and live plants together. You can find a list of plants that are suitable for adding to driftwood in this article: Aquarium Plants That Grow On Rocks And Driftwood.

Using A Reverse Osmosis Filter

A reverse osmosis filter will remove many of the dissolved minerals from water. Reverse osmosis filters work by passing water through a semipermeable membrane.

Because reverse osmosis is so effective, it will purify the water until almost no dissolved minerals are left. The process can also remove many of the beneficial minerals and bacteria from water essential to a fish’s health.

Can you over filter a fish tank using reverse osmosis? Reverse osmosis is a form of over-filtering so it should only be used occasionally and when pH levels are very high or low, and you leave some water unfiltered to retain some of the essential minerals, otherwise you will end up with low alkalinity in the fish tank.

Aquatic Life Reverse Osmosis Buddie

Aquatic life reverse osmosis buddie for 50 gallon aquariums
Aquatic Life RO Buddie

The Aquatic Life Reverse Osmosis Buddie is the perfect way to remove minerals and heavy metals from your aquarium water, which in turn will provide a more neutral pH level.

Although compact, and easy to store, this unit is suitable for aquariums of around 50 gallons.

The aquatic life RO Buddie comes with sediment and carbon cartridges as well as a mixed bed resin cartridge, which removes the remaining total dissolved solids from the water.

Using A Chemical Filter

Chemical filters work by binding to dissolved minerals and removing them from the water. They are often used in conjunction with a reverse osmosis filter to remove dissolved minerals.

Chemical filters can be an effective way to lower the alkalinity in a fish tank. Choosing a safe chemical filter for aquarium use is vital as some can be harmful to aquarium fish.

When using a chemical filter, you must carefully follow the manufacturer’s instructions.

Add White Vinegar Or Lemon Juice

By adding any acidic compound to the water, it will have the effect of lowering the pH. Over time, acidic compounds will erode the buffering minerals present and reduce alkalinity, but this is not immediate, whereas a drop in pH will be quick.

A drop in pH caused by adding acidic compounds such as vinegar or lemon juice is temporary, and you will find it harder to lower the pH because of the buffering minerals acting on the acids.

When adding lemon juice or vinegar, you will need to experiment. Start by adding small amounts and measuring the pH frequently but remove any aquatic life from the tank beforehand.

What If Alkalinity Becomes Too Low

Low alkalinity in fish tank water can cause some instability as the water will lack the minerals that act as buffers against pH swings. If you have managed to lower your aquarium’s alkalinity too far, you will now have a tank that is too acidic which is the opposite problem.

A slightly too high or low pH will not cause too many problems overall. If you have gone much too far and you need to raise the alkalinity of your aquarium water quickly, you can use a chemical treatment such as sodium bicarbonate (baking soda).

Baking soda has a natural pH of 8 and will act quickly in raising the alkalinity, so you will need to remove your fish to protect them from sudden changes.

The general rule when adding baking soda is 1 teaspoon per 5 gallons of water. This will have an immediate effect but will take time to work through the whole tank.

If you are struggling to balance the water parameters in your aquarium, you may need to perform an emergency cycle to get things back in check. Fish tank cycling generally deals with the biological cycle (or nitrogen cycle) where healthy bacteria keep ammonia and nitrites at bay, but these can also affect the pH so a simple cycle may get you back on track.

Find out more about cycling your fish tank in the articles below:

How to cycle a fish tank (Step by Step).

How To Cycle A Fish Tank In 24 Hours (Emergency Cycling).

Can Low Alkalinity Kill Fish

Low alkalinity is unlikely to kill your fish, but it can cause sudden pH swings which can kill fish that are sensitive to these sudden changes. Some fish suffer a form of shock when water parameters change suddenly which leads to stress and sometimes death.

Low alkalinity in fish tank water usually means a lack of essential minerals that are vital to the health of your fish, so long-term exposure to this type of water will have a negative effect on the overall health of your fish.

Is High Alkalinity Bad For Fish

High alkalinity in itself is not necessarily bad for fish, but it can be a symptom of other water quality problems.

Yellow Peacock Cichlid From The Alkaline Waters Of Malawi Lake (East Africa)
Yellow Peacock Cichlid From The Alkaline Waters Of Malawi Lake (East Africa)

Overly alkaline water can indicate that there is too much dissolved calcium in the water, which can lead to health problems for fish.

Additionally, high alkalinity can make it difficult for fish to regulate their body temperature, making them more susceptible to stress and disease.

As a result, it is essential to monitor pH and alkalinity levels in your fish tank by taking a regular water sample that can be tested. Once you have detected fluctuating pH levels, you can then take steps to keep them within a healthy range.

If you have any concerns about your fish’s health, consult with a veterinarian or qualified aquarium specialist.

Will High Alkalinity Kill Fish

It’s a common misconception that high alkalinity will kill fish. In reality, high alkalinity can be beneficial to fish, providing a buffer against pH swings and helping maintain a stable environment.

However, there are some circumstances in which high alkalinity can be harmful to fish. If the water is already hard (high in minerals), adding salt can increase the hardness and make it difficult for fish to regulate their body fluids and temperature. In addition, elevated alkalinity can cause problems for fish that are sensitive to sudden changes in pH.

As a result, it’s important to be aware of your aquarium’s alkalinity levels and take steps to keep them within a safe range for your fish.

What Causes High Alkalinity In A Fish Tank

High alkalinity in a fish tank is quite common and there are several different factors that can contribute to this problem.

The most common cause of high alkalinity in a fish tank is simply using tap water to fill the tank. Tap water usually contains high levels of minerals, which can quickly raise the alkalinity of a tank.

Another common cause of high alkalinity is overfeeding. When fish are overfed, they produce more waste, which raises ammonia and nitrites in the water; this, in turn, raises the pH level and increases alkalinity.

Fish That Like High Alkalinity

Some fish species prefer water with high alkalinity, and some prefer a low alkalinity (or acidic) environment. Most fish can adjust to differing water hardness and pH when the changes are gradual, but they will each have a preferred pH and water hardness level based on their natural habitat.

The types of freshwater fish that prefer a high alkaline environment will generally come from areas such as Central and South America or The Great Lakes Of Eastern Africa. These areas are high in such minerals as phosphates, borates, and limestone, which dissolve into the water, causing it to become hard with a natural buffering ability against acids.

African Cichlids Prefer High Alkaline Levels
African Cichlids Prefer High Alkaline Levels

Suppose your tap water is naturally high in minerals. In that case, it will be harder water and have a higher alkalinity level, so it may be easier to stock your aquarium with fish used to this type of water instead of constantly treating it to reduce the level of alkalinity.

Some popular aquarium fish for a high alkaline, hard water environment are:

  • African cichlids
  • Many Central American Cichlids
  • Mollies
  • Platies
  • Swordtails
  • Guppies
  • Danios
  • Goldfish
  • Dwarf Loach
  • Clown Loach

The above is just a selection of many fish that will thrive, so you would be advised to research each species beforehand. Another critical factor is that your fish may have already become accustomed to softer, more acidic water throughout its captivity.

I mentioned previously in this article that many fish will adapt to most environments as long as the changes are not sudden. There are a few exceptions, such as freshwater discus, which are very delicate and will need software to thrive.

Wrap Up

Throughout this article, I have not only covered some ways that you can lower alkalinity in a fish tank, but I have explained the importance of alkalinity as a buffering factor to protect from big swings in the water’s pH.

The relationship between pH and alkalinity is close as they directly impact each other. Most fish can adjust to water that has a high level of alkalinity, but fish need the water chemistry to be as stable as possible, and this is more likely when the buffering capacity is higher.

Very high levels of alkalinity can become a problem for the health of your fish, so you will need to reduce the level of alkalinity if this is the case.

Frequently Asked Questions

Jon O'Connell

I have kept both marine and freshwater fish and set up almost 100 aquariums. Although I am no longer doing it professionally, I am happy to share my knowledge and experience to help others enjoy keeping healthy and happy fish.

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