The pH value in water is a considerable fact in fish keeping. Fish need an optimum pH range for a better life. But in some cases, the pH range fluctuates from the optimum range. What causes high pH in aquariums? Let’s clarify it.

Generally, fish can live in the pH range of 6.5 to 9 without harm. But their blood pH level is around 7.5. Therefore, it’s better to keep your aquarium pH value around 6.8 to 7.9.

Frequent pH fluctuations push the fish into stress. It can be a reason for low survival rates, breeding errors, and deaths.

What causes high pH in aquariums?

Untreated water (water hardness)

Water hardness is a prominent reason to causes the initial high pH values in an aquarium. Water hardness means mineral levels (amount of dissolved calcium and magnesium in the water). Dissolved calcium and magnesium cause to increase in the water pH level significantly.

According to chemistry, the pH level in pure water is 7. But we can’t find such pure water in our environment. Available water consists of irons that can change the pH.

Generally, aquariums are filled with tap water, spring water, or groundwater. Some people use boreholes also. These sources all originate from groundwater. Groundwater comes across the soil or bedrock that contains chemical components. These contaminants (minerals) dissolve and collect in groundwater, which causes high pH.


Reverse osmosis or de-ironizing can remove the contaminated irons and reduce the pH. It produces high-purity water with low hardness. Fill 50% of the RO water into your tank.
A commercial water softener removes the minerals in hard water and reduces the pH. Treat the water before adding it to the aquarium.

Rocks and stones

Some rocks are rich in minerals that can dissolve in water gradually. In particular, Limestone, corals, and shells comprise calcium components. It can increase the pH level significantly by dissolving aquarium water. But it is not happening in a single day; it takes time.

Do you want to know how to clean your aquarium rocks?


Regular water changing will remove the minerals, but frequent pH monitoring is required. Or else, be careful not to use calcium-rich minerals.

Lack of Dissolved CO2

Carbon dioxide dissolves in water and makes acidic components. Therefore, low CO2 causes an increase in the pH value. This can happen in planted aquariums, where plants need CO2 for photosynthesis. Also, highly aerated aquariums face a low CO2 problem.


Some people supply CO2 externally as required. Usually, this happens in planted aquariums.

Read more..
How To Lower pH In Aquarium Naturally? Eco-Friendly Practice

I am Bruce Daniel Holgate from Los Angeles, the founder of, environmental scientist and content writer. Researcher and hobbyist in aquarium keeping and fish breeding with 20 years of experience. I'm willing to share my knowledge with you all.


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