What Causes High pH in a Fish Tank? – 5 Reasons & Solutions


Having an aquarium is both a privilege and a responsibility. And as responsible fish tank owners, cleaning the water is a regular task, and maintaining the pH level in the aquarium is vital. But sometimes, we overlook things and face a fish tank pH level that is too high.

So what causes high pH in a fish tank? The absence of carbon dioxide and chemical substances leaching out can cause the pH to rise in the aquarium⁠. Here is a list of common culprits you can easily miss out on–⁠ tap water, filters, plants, and substrate.

Reasons for High pH Alkalinity in a Fish Tank


1. Source water


Having an aquarium requires regular maintenance. And one of the routine tasks you must do is replace the water. Most people use tap water, which can sometimes be the reason why there is high pH in the aquarium.

The source water or tap water used for your aquarium may vary in pH due to its composition. So it is essential to know the initial pH of your tap water. You can test the pH by using an electronic pH meter which can be bought in hardware stores and wait for the results.

2. Filters


Flawed or broken filters probably cause high pH in your fish tank. The filtration system might be sweeping off waste materials poorly, worsening the water condition and causing it to raise alkalinity in the fish tank.

A good-quality filter is vital to a long aquarium’s lifespan since it extracts substances and harmful pollutants from the fish tank. And, cheap filters break easily, causing detrimental waste to accumulate over time, leaving you with high levels of ammonia and pH.

3. Plants and algae


Plants and algae are known absorbers of carbon dioxide, so the excess presence of aquatic plants might increase the pH in the aquarium.

However, completely removing all the plants and algae inside is detrimental because there are benefits, such as pet shelter and oxygen provision. So, maintaining enough plants and algae is better.

4. Substrate


The substrate might be what causes high alkalinity in an aquarium. The soil type you use might affect the aquarium’s pH levels because some soils dissolve and introduce new chemicals, and some do not.

Choosing an inert substrate–⁠one that does not cause changes to the pH–⁠is generally safe. Gravel and sand are inert substrates commonly used. But it is worth noting that other substrates can benefit the aquarium, depending on the fish you have.

5. Aeration


Providing good circulation for your fish to breathe through aeration makes them happy. But too much aeration can cause the fish tank’s pH levels to increase.

Too much oxygen circulation can also mean a rapid depletion of carbon dioxide, which produces carbonic acid. So, less acid, more alkaline–⁠high pH–⁠kill fish.


1. Peat moss


Placing peat moss can lower the pH level in the fish tank. These are degraded fibrous materials that occur in bogs, but you can buy them in pet stores.

The peat moss releases tannins, which remove dissolved bicarbonates. These said components are usually responsible for increasing the pH level.

2. Driftwood

Setting up driftwood is also an excellent solution to lower pH in a fish tank naturally. Driftwoods are small pieces of wood usually washed ashore from lakes or rivers. It functions the same way as peat moss–⁠and more.

Driftwood’s other benefits are that it creates a natural aesthetic and improves the aquarium’s overall appearance.  It can also be a shelter and a food source for your pet fish.

3. Catappa/Almond leaves

The catappa leaves are also tannin producers, which can ⁠fix the high pH in a fish tank. You can also buy these leaves from pet shops.

Another good thing about the catappa or almond leaves is that they release other beneficial chemicals for your fish buddies’ health. You can utilize these leaves if you want to keep the fish healthy.

4. Reverse osmosis system

The Reverse Osmosis (RO) system keeps the water pure by filtering out all unwanted chemicals and impurities, even the smallest ones. As a result, it keeps the pH level neutral.

However, a fancy-sounding device like the RO filter is costly. That said, it is worth investing in if you want to balance the pH level while keeping control of the substances in the aquarium.

5. Aquarium additives

Another alternative is to get aquarium additives, which can help lower the pH of the aquarium. Different products lower pH in varying amounts, so research to keep your fish safe from harm before buying.

Importance of pH Levelwhat-causes-high-alkaline-in-an-aquarium

According to the pH level chart, 0 to 6.9 is acidic, 7.0 is neutral, and 7.1 to 14 is alkaline. The ideal environment for a fish tank is a pH level of 6.8 to 8.0. If it goes lower or higher for prolonged periods, issues may arise.

You must maintain the pH level close to neutral as consistently as possible, as it keeps your pets away from illnesses, such as alkalosis and acidosis. Even quickly lowering the pH in the aquarium can introduce fatal diseases and stress to the fish if neglected for too long.

Certain fish might thrive better from lower or higher pH levels, so it is best to know what environment your fish requires. Knowing what your pets need before harm comes their way can save you from trouble and expenses.

How to Know if the pH is High in the Aquarium?

You need to watch out for the symptoms of high pH in the fish tank. These symptoms may show in the fish or even in the environment.

Fish symptoms you can look out for are whether they:

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  • behave frantically
  • swim back and forth
  • try to jump out
  • try to scratch themselves against stones
  • have spread-out fins
  • have gills with mucus.


A typical environmental symptom is the speedy growth of algae around the aquarium, showing on walls and stones. When the fish tank turns green and slimy, that is a clear sign of high pH.

How Often Should I Test the pH in the Fish Tank?


A good rule of thumb is to have a consistent schedule for testing the pH level of the fish tank. You can do it monthly, but biweekly is ideal to be safer.

Changes in the aquarium mean testing the pH level. Some changes are:

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  • Adding new plants or stones
  • Changing the tap water
  • Putting aquarium treatments
  • If the fish gets sick.



There are many reasons why you would want to learn what causes high pH in a fish tank. Hopefully, you are reading this to prevent future problems. But if that’s not the case, let this article be a tool for you to avoid any fatal issues in the aquarium.

We hope you find this article helpful if you ever face this challenge and gain a deeper appreciation for maintaining an aquarium.