Symptoms of High pH in Fish Tank- 6 Signs With Solutions

Yesterday my friend was explaining some problems in his fish tank like fish gasping for air, stress, poor appetite, damage in fins, and sluggishness in fish and algae bloom. He was too upset seeing the condition of his fish tank and asked me to sort out his problem.

Following that, I asked him more questions. Then, after getting proper information about those problems, I marked them as the symptoms of high pH in his fish tank. As I have already faced the same problems last year, I was clear about what to do.

Then, I suggested to him the solutions. If you are also going through the same issue, you may also check the ways that I mentioned here. Hopefully, it will be helpful for you.

6 Symptoms Can Make You Understand About High pH in Fish Tank

1. Fish Gasping for Air

The most noticeable symptom that I found in my fish due to high pH was respiratory distress of the fish. At the beginning of my aquarium experience,  I didn’t know much about pH measurement and then faced these problems.

When the pH levels in the water are too high, fish may begin to gasp for air at the surface of the tank. This is because high pH levels can lower the amount of oxygen in the water, making it harder for fish to breathe.

Fish breathe through their gills, which are specialized organs that extract oxygen from water. In high-pH water (i.e. water with a high concentration of hydroxide ions), the water’s capacity to carry dissolved oxygen decreases.

As a result, the oxygen concentration in the water decreases, making it difficult for fish to obtain the oxygen they need to survive. When fish are unable to obtain sufficient oxygen through their gills, they may start to gasp for air at the surface of the water. This behavior is a sign of respiratory distress.

2. Agitation or Stress in Fish

Not only breathing problems, but I found my fish to be stressed also due to high pH. High pH levels can also cause fish to become agitated which can lead to erratic swimming, jumping, or even aggression towards other fish in the tank.

Fish can become stressed in high pH (i.e., basic or alkaline water) because it can disrupt their internal balance of ions, which is crucial for their bodily functions. Fish maintain a delicate balance of electrolytes, such as sodium, potassium, and calcium, in their bloodstream and tissues.

This type of mechanism is essential for their metabolism, osmoregulation, and overall health. When the pH of the water increases, the concentration of hydrogen ions (H+) decreases, and the concentration of hydroxide ions (OH-) increases.

However, this shift in ion balance can make it difficult for fish to regulate the balance of electrolytes in their bodies, leading to physiological stress and potentially lethal conditions. For example, it can cause alkaline burns on the skin and gills and damage to the nervous system, resulting in behavioral changes, loss of coordination, and even death.

In summary, fish become stressed at high pH because it disrupts their internal balance of ions, which can negatively impact their metabolism, osmoregulation, respiratory function, and overall health.

3. Sluggishness in Fish

Also, I noticed sluggishness in my fish when my tank pH increased too large. High pH levels can also cause fish to become lethargic and sluggish. This is because high pH levels can interfere with the fish’s metabolism and affect their ability to move around and function normally.

As you know, fish are sensitive to changes in their environment, including changes in water chemistry.

When the pH of the water is too high, it can cause an imbalance of ions in the fish’s body fluids, leading to a condition known as alkalosis.

This can cause the fish to become lethargic or sluggish, as it disrupts the normal function of their nervous system and muscles. Additionally, high pH can also affect the levels of dissolved oxygen in the water, which can further exacerbate the fish’s sluggishness.

4. Algae Blooms

Further, high pH levels can also promote the growth of algae in the tank. This can lead to cloudy or green water and can also compete with fish for oxygen and other nutrients.

As you know, pH can affect the growth of aquatic plants and animals, including algae. In a fish tank, a high pH level can promote the growth of certain types of algae, such as blue-green algae (cyanobacteria).

This is because these types of algae thrive in alkaline environments and a high pH level provides them with the ideal conditions for growth. When algae blooms occur, they can quickly take over a fish tank, covering surfaces with a green or brownish layer.

Moreover, it also initiates depletion of the oxygen levels in the water, which can be harmful or even deadly for fish. That’s why to prevent algae blooms in a fish tank, it’s important to maintain a stable pH level and keep it within the appropriate range for the fish and plants in the tank.

However, algae bloom can also be a sign of high calcium or high phosphate level in a fish tank. You can also check the symptoms and the way of lowering those minerals from my previous content published on this website. Here is the Link for you-

  1. How to Redecue Calcium Level from Aquarium
  2. How to Get Rid of Phosphates in Aquarium

5. Poor Appetite

Fish may also exhibit a poor appetite or refuse to eat altogether when the pH levels in the water are too high. In a fish tank, a high pH level can cause poor appetite in fish because it can disrupt the chemical balance and biological processes within the fish’s body.

Fish have a specific pH range in which they thrive and any significant deviation from this range can cause stress and discomfort, leading to a loss of appetite. A high pH level in the water can also cause an increase in the concentration of certain ions, such as ammonia and nitrite.

Increasing these elements can be toxic to fish at high levels. These toxins can irritate the fish’s gills, leading to respiratory problems and a decreased appetite. Additionally, a high pH level can affect the availability of certain nutrients that are essential for the fish’s growth and overall health.

For example, a high pH level can limit the availability of iron, which is necessary for the production of hemoglobin, a protein that transports oxygen in the fish’s blood. This can lead to anemia and a decrease in appetite.

6. Damaged Fins

High pH can damage the delicate fins of fish, leading to frayed or torn fins. High pH levels can cause damage to fish fins in a fish tank for several reasons.

Firstly, when the pH of the water in a fish tank is too high, it can lead to the accumulation of ammonia and nitrite, which are toxic to fish. These toxins can damage the delicate tissues of the fins and other parts of the fish’s body, leading to fin rot, necrosis, and other health problems.

Secondly, high pH levels can cause mineral deposits to build up on the fins of fish. This can cause the fins to become stiff and brittle, making them more prone to breaking and tearing. In severe cases, the fins may even become calcified, which can be very painful for the fish.

Thirdly, high pH levels can also lead to the growth of harmful bacteria and fungi in the water, which can cause infections in the fins and other parts of the fish’s body. These infections can cause the fins to become red, swollen, and inflamed and can lead to serious health problems if left untreated.

Consequently, to prevent these issues, it is important to maintain a stable and appropriate pH level in the fish tank, usually in the range of 6.5 to 7.5, depending on the species of fish being kept. 

How to Fix High pH in Fish Tank

Test the pH level

As high pH is detrimental for fish and other aquatic inhabitants, you should take the initiatives to control it immediately after identifying the problem. In this case, your first step should be to test the pH level using a pH test kit. This will give you an idea of how high the pH level is and how much it needs to be lowered.

Partial Water Changes

Next, you can partially change the water in your tank, replacing it with fresh water that has a lower pH level. But, make sure to use a water conditioner to remove any chlorine or other harmful substances from the tap water.

Use peat moss or driftwood

If you don’t want to change the water, you can add peat moss for driftwood to your tank. It can also help to lower the pH level. These natural materials release tannins that can lower the pH level over time.

Use a Commercial pH Decreaser

Next, You can choose a commercial pH decreaser, which is available at most pet stores. Follow the instructions carefully, as too much can cause a sudden drop in pH, which can be harmful to your fish. (Our pick: Aquarium Pharmaceuticals 30B pH Down Aquarium pH Adjuster)

Add Live Plants

Many hobbyists ask me, “Will Plants Lower pH in Aquarium?”. The answer is, ” Yes!”. Live plants can help to absorb excess nutrients and lower the pH level in your tank. Some good options include Amazon swords, java moss, and hornwort.

Check your Aquarium Gravel

Finally, you can think of some aquarium gravels. If you have alkaline gravel or substrate, it can be contributing to the high pH level in your tank. To facilitate the process, consider replacing it with a neutral or acidic substrate.

Remember to test the pH level regularly and make adjustments gradually. Sudden changes in pH can be harmful to your fish.

What Causes High pH in Fish Tanks

Water Hardness

The simplest but most problematic fact is that hobbyists don’t understand the causes of high pH in their fish tanks. As a result, though they try to maintain the ways of lowering pH, it doesn’t remain effective for a long time.

For example, high levels of minerals such as calcium and magnesium in the water can lead to increased pH levels. But, many hobbyists don’t know about this. This is especially common in areas with hard water. But they think that those minerals have a positive impact on the tank.

Overuse of Chemicals

Again, using too many chemicals such as pH adjusters, water conditioners and medications can cause a sudden increase in pH levels.

Biological Factors

Sometimes biological factors may also be responsible for increasing pH. High pH levels can be caused by excessive biological activity in the tank, such as an overabundance of algae, bacteria, or other microorganisms.

Lack of Aeration

Insufficient aeration can cause pH levels to rise due to the buildup of carbon dioxide in the water.


If the tank is contaminated with substances such as cement or lime, this can lead to a rise in pH levels.

Can High pH Kill Fish?

Yes, high pH levels can be harmful and even fatal to fish. Though different species of fish have different preferred pH ranges, the alkaline situation is disastrous for most fish.

When the pH of the water becomes too high (alkaline), it can disrupt the natural balance of the fish’s internal fluids. This can lead to various health problems and even death.

Most freshwater fish species thrive in water with a pH between 6.5 and 7.5, while saltwater fish prefer a pH between 8.1 and 8.4. However, if the pH level of the water rises above these ranges, it can become stressful and eventually fatal for the fish.

For example, a high pH level can cause ammonia to become more toxic, which can lead to ammonia poisoning in fish. Additionally, a high pH level can interfere with the fish’s ability to absorb oxygen, causing suffocation.

Ideal pH Range for Fish Tank

Check the ideal pH range for your fish tank to maintain the level properly-

Water tankIdeal pH measurement 
Reef tank8.0-8.4
Saltwater aquarium 8.0-8.4
Freshwater aquarium 6.5-7.5
Brackish aquarium 7.5-8.4
Planted aquarium 6.5-7.5
Nano aquarium6.0-7.5
Coldwater aquarium 6.5-7.5


How can I test the pH level in my fish tank?

You can test the pH level in your fish tank using a pH test kit or a digital pH meter(Our pick: PH Meter for Water Hydroponics Digital PH Tester). These tools will give you an accurate reading of the pH level in your tank.

Can high pH harm plants in my fish tank?

Yes, high pH can harm plants in a fish tank. Just like fish, plants have specific pH requirements and a pH that is too high or too low can cause damage or even kill them.

Why is it important to maintain the proper pH level in a fish tank?

Maintaining the proper pH level in a fish tank is important because fish and other aquatic organisms have specific pH requirements to survive and thrive. If the pH level is too high or too low, it can cause stress and health problems for the fish, leading to sickness or death.

Final Thoughts

In summary, high alkalinity in a fish tank can negatively impact the health and well-being of fish, leading to several health problems. I suggest to my friends and neighbors to monitor and maintain the pH level within the appropriate range for the specific species of fish in the tank.

Otherwise, it will be tough to ensure optimal health and growth. You can get ideas about how to lower the pH in a fish tank naturally and by using other mediums. You have to just have an idea about what is High pH in a fish tank and notice your aquarium’s condition properly. Thus, you can maintain your tank properly.

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