What Causes High pH In Aquarium & How To Combat The Problem (8 Techniques)

As you may think, High pH is terrible, but it’s not bad for all because some of the fish, including African Cichlid, Plecos, love that. Your aquarium pH should be in accord with your fish preference. Most importantly, you need to keep your aquarium water pH stable.

Problems arise when your pH fluctuates too much or changes in quick time. Fish get stressed when a vital parameter like pH alters suddenly. But what causes high pH in an aquarium?

The water pH varies for several reasons, including chemical concentration, minerals, types of substrate, etc. So, let’s explore what raises pH in an aquarium and how to adjust pH.

What Are The Effects Of High pH In The Aquarium?

High alkaline water (high pH) affects fish gill’s, which is the organ fish use to get dissolved oxygen from aquarium water. You can compare it with your lungs. Fish may not get oxygen to survive if their gills are not fully functional. In other words, fish may die because of high pH. 

High pH can damage the protective layers (mucus) that cover the fish body. It can also affect the fish’s tails, fins associated with troubled growth. Keep in mind, water high in pH is much more dangerous than low pH. 

High water pH also increases the risk of ammonia toxicity in the aquarium. “Ammonia auto-intoxication” can occur when water pH is higher than the blood pH of fish. It happens mainly when water pH rises above nine and fish experience higher ammonia levels in their blood. 

Causes Of High pH In Aquariums?

Causes Of High pH In Aquariums

1) In aquariums, substrate (rocks, stones, crushed corals, etc.) and decors containing calcium-rich limestones can be a reason for high pH. Before putting anything in your aquarium, make sure those are chemically inert and can’t alter your pH. 

2) A low level of CO2 in the water is another cause of high pH. Increased aeration makes CO2 escape from the aquarium water; CO2 in contact with water forms weak carbonic acid, lowering pH. 

3) Tap water that you are using in your aquarium could be a reason for high pH. If it contains alkaline substances, the pH will be higher. You can check your tap water pH. To get the actual pH of your tap water, you have to measure it after aerating the water for 24 hours in a bucket.

4) Failure to perform the routine filter maintenance, in other words, a high pH, may be observed in an improperly maintained aquarium. Accumulated fish excreta and other wastes can increase the pH level.

5) Algae can raise the pH level even in well-buffered water. When lights are on, they produce a massive amount of dissolved oxygen during photosynthesis, which forces CO2 away and raises the pH. 

6) In a planted aquarium, CO2 and nitrates (as a food source), which lowers the pH consumed by plants, raise the pH. So, if you have plants ensure you have the right balance of CO2 and nitrates. 

How To Lower The pH Naturally

You can easily adjust your pH by the chemicals available in the market. However, this is not a sustainable solution. Chemicals will lower the pH rapidly, but that will not be stable. Instead of wasting money buying chemicals, I’ll suggest using any of the natural methods below.

Peat Moss

Peat Moss to lower aquarium water pH

Using peat moss is a common way of lowering the pH. Peat moss has a slightly acidic property that helps to reduce the pH. However, one disadvantage is it can bring a yellowish tinge to your aquarium. You can pre-treat separately before adding in the tank to lower down the yellowish impact.

Peat moss needs to stay in your tank for best results; you can place peat moss in your filter or substrate. How much should you use? There’s no exact measurement. It depends on your requirement; however, moderation is key. Start with a small amount and adjust the quantity based on the result.


Driftwood to lower aquarium water pH

Driftwood addition to lower the pH is a good idea since it can also bring aesthetics to your aquarium. Like peat moss, it releases tannins and makes aquarium water yellow. 

You can’t put driftwood directly into the aquarium. You have to prepare that well; otherwise, any harmful contaminants present can adversely affect your aquarium. Check my detailed article on how to prepare driftwood most efficiently.

Almond Leaves

Almond Leaves to lower aquarium water pH

Almond leaves also work the same way as driftwood. Not only does it lower the pH, but it also has some antibacterial properties, which works as an added benefit if your fishes are suffering from fin rot or you are raising vulnerable fry.

If you don’t want to clutter your aquarium with leaves, you can soak the leaves separately; when water gets sufficiently stained, you can use that water in your aquarium.

RO Water

RO Water to lower aquarium water pH

RO water is neutral in pH; changing small portions of aquarium water with RO water will help to lower the pH. But keep in mind you don’t want to reduce the pH drastically. So, change a small amount first, see the effect, then take action accordingly.

Reverse Osmosis effectively removes impurities, arsenic, and heavy metals from the water and maintains a stable neutral pH.

But the RO unit is costly; if your tap water is high in pH and you want to keep fish ideal for a neutral pH range, you can consider buying an RO unit for a permanent solution.

CO2 Addition

Co2 to lower aquarium water pH

CO2 addition is another way to increase the pH. If you have a planted aquarium, you can consider introducing a CO2 system.

The CO2 system helps in plant growth and will play its part in lowering down the pH. However, this option is only feasible if you have a planted aquarium. Check my article to set up a complete CO2 system for aquariums

Addition Of Vinegar

Vinegar to lower aquarium water pH

Adding vinegar (white vinegar, 5% acetic acid) can instantly reduce the pH. But you shouldn’t lessen that immediately.

I suggest using 01 ml of vinegar per gallon of water to decrease 0.3 units of pH at a time. Going slow is recommended for this procedure rather than proceeding too fast. Don’t use wine vinegar since it may contain unwanted organics.

Maintaining The kH

Maintain kH to lower aquarium water pH

KH is a measure of bicarbonate and carbonate ions in the water. Although people mistakenly redirect kH as alkalinity, that’s not correct; kH is the measure of the buffering ability of water.

A healthy kH level will prevent a sudden swing of pH in your tank. Low KH level, especially when it goes below 4.5° dH, a significant change in pH may occur; therefore, you have to monitor kH level closely.

Although It’s not directly a solution, it will prevent you from leading to a problem, and who doesn’t believe the cliche, prevention is better than cure.

You can read my article on softening aquarium water, which is relevant to this purpose.

Reduce Algae Growth

Algae to lower aquarium water pH

Algae contribute to higher pH during photosynthesis. Algae appear due to low CO2 levels, excess nutrients in the water, and too much light. Regulate these three things in your aquarium to keep algae growth always in check. Check my article on how to prevent algae

Monitor pH Level And Keep It Stable

The vast majority of freshwater fish like almost neutral to slightly high pH (7-8.5 range). Different fish adapts to different pH levels; what works for one might not work for others.

So, before setting the pH level, make sure you know exactly what your fish need. Fish thrive when your tank water pH is similar to their natural habitat.

You can’t keep fish together in the same tank; that has a vast difference in their pH requirement.

Ideal pH for aquariums

Image Source: https://www.aquaculturealliance.org

After adding the fish and knowing their pH need, you have to monitor the pH regularly. In order to test, you need to buy a test kit, which you can get from a local pet store or online.

I recommend purchasing an API test kit, it’s reliable, and you can also test other water parameters as well with this kit, including ammonia, nitrate, etc. 

You don’t need to chase the perfect pH. What is necessary is to keep the pH stable and steady. A drastic change in pH imposes stress on fish.

Therefore, you have to avoid too much fluctuation. The best way to prevent pH change is to improve the buffering capacity for your aquarium water.

Besides, a routine partial water change and maintenance will help you keep water’s pH stable.

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Final Thoughts

Aquarium pH is one of the significant parameters to keep your fish healthy and happy. Extreme pH reduces feeding activity, hinders growth, and hampers fish’s immune system.

You already know what causes high pH in aquariums; therefore, you should first find out the reasons if you notice pH raises. After that, take steps to adjust the pH, don’t make any prompt, take time and gradually lower the pH to your desired level.

Sujit Modak

2 thoughts on “What Causes High pH In Aquarium & How To Combat The Problem (8 Techniques)”

  1. Thanks for the fine article. Question: what causes low pH? I ask because I have a tank with mystery snails. They need a pH of 7, but the pH in my tank is consistently on the lower end. Any idea on what might cause that? I have purchased a powder which will raise the pH to 7, but I’m not sure it lasts long especially if something in the tank is causing low pH?
    The tank in question is small, 10 gallons. I have some guppies, which I monitor to protect the snails. Some guppies are too aggressive so I move them to a smaller tank. Too many fish are not good for these snails. But they seemed to do okay for a while, they die off for no apparent reason. Hence my curiosity and work on pH.


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