Best Methods To Safely Lower pH In Your Aquarium

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The pH levels in your fish tank can have a serious effect on your fishes’ health, resulting in symptoms of ill health and even mortality when especially sensitive species are kept in the wrong conditions.

So, what causes high pH in an aquarium? And why is having the correct water chemistry so important for your fish’s well being?

Read this guide to find out how to lower the pH in your aquarium and how to raise the pH in the aquarium, too.

What Is pH And What Is The pH Scale?

The pH of water refers to water chemistry. Essentially, the pH is dependent on several factors, including the presence of trace minerals, chemical concentration, and the kind of substrate you have in your tank.

Water pH is measured on a scale ranging from 0 to 14. On the scale, a pH level of 7.0 is regarded as neutral. Values below 7.0 are acidic, whereas those above 7.0 are alkaline.

The pH Scale mini infographic

All fish species are adapted to different pH levels, depending on what’s found in the fish’s natural environment. So, whereas low alkalinity might be good for one fish species, it might not necessarily be tolerable for another. That’s why you must always research the preferred water parameters of every fish species you’re planning on adding to your tank. 

If the fish aren’t compatible with their pH requirements, there’s a good chance that some of your fish will be stressed and will most likely become sick or even die.

How pH Impacts Other Aspects Of Water Chemistry

Water pH can impact several other aspects of the water chemistry and water quality in the aquarium.

How so?

Well, if the pH falls below 6.0, the nitrifying bacteria that remove the toxic ammonia and nitrites from the water in your fish tank start to die off. That makes ammonia spikes highly likely, which often results in mass fish kills. Also, the toxicity of ammonia is mostly dictated by the pH.

Litmus strips for measurement of acidity.Beaker with water

Ammonia And pH

The ammonia in your aquarium is formed by a combination of ammonium ions, NH4+ and ammonia, NH3.

The water pH is highly influential in the concentrations of those two ammonium compounds. In alkaline water, where the pH is above 7.0, more ammonia will be present. In acidic water, where the pH is below 7.0, there will be more ammonium ions. Of the two compounds, ammonia is the most toxic to your fish.

So, it’s crucial to know that you are making the environment more toxic for your fish if you raise the pH. For that reason, I don’t recommend adjusting the pH while you’re cycling your fish tank. Of course, once the cycle has completed, you shouldn’t detect any ammonia in the water.

How To Test The pH Of Tap Water

Most hobbyists use tap water for their fish tanks. Tap water has a pH, which can vary, depending on the region, or even the city, where you live. So, how do you know the pH of your tap water?

  • First of all, don’t test your tap water pH immediately you get the water from your faucet.
  • To get the “true” pH, run some water into a bucket, and use an air stone to oxygenate the water.
  • Leave the bucket to “rest” for 24 hours.
  • Now, test the water pH, and make a note of the reading.
  • Test the water again after a further 24 hours to see if the pH has changed.

The reading that you get after 48 hours will be the true pH of your tap water.

Why Let The Tap Water “Rest?”

So, why is it necessary to allow the water to rest for 24 to 48 hours?

Well, that’s because carbon dioxide in the water makes the pH level fall. If you trigger gaseous exchange by agitating the water surface with an air stone, you draw oxygen from the air into the water and push carbon dioxide from the water into the air.

That reduction of carbon dioxide in the water causes the pH to rise. That pH will be the true level that you’ll get in your aquarium since the oxygen/carbon dioxide exchange is a constant process in the fish tank environment. Of course, that’s assuming you don’t have any other chemicals or objects in your tank that will influence the pH.

What’s The Relationship Between pH And kH?

Male hand holding PH tests in front of freshwater aquarium.

The kH level in your fish tank measures the concentration of bicarbonate and carbonate ions in the water. The kH is also an indicator of the water’s ability to neutralize or buffer acids without significantly altering the pH. So, the higher the kH, the more stable the pH in your fish tank will be. A lower kH correspondingly causes more dramatic swings in the pH value.

So, if the kH is under 4.5odH, you must watch your pH for large alterations. Your water changes must also be very consistent since the low kH will continually fall over time. In these circumstances, performing frequent partial water changes is the most effective way to keep the pH within a suitable range.

What Is The Ideal pH For An Aquarium?

A pH of 7.0 is regarded as neutral. Most freshwater tropical aquarium creatures prefer a pH in the range of 6.8 to 7.6, although you should research the fish species you want to keep, as some need lower or higher levels.

Check Your Test Kit!

The most accurate way to monitor the water pH in your fish tank is to use a high-quality, reliable aquarium water test kit.

Test kits are available as simple dip tests or solutions. In either case, you compare an aquarium water sample reading with a color chart that comes with the kit. The color of the water equates to the pH level shown on the color chart. The main drawback to using these kits is that they can be inaccurate, especially if you don’t follow the instructions to the letter!

Also, kits have a typical shelf life of around six months. If you have a test kit that’s older than that, the results might be inaccurate.

To be on the safe side, carry out two tests, making sure that you follow the instructions carefully. If you’re still not confident in your results, most good fish stores will test your water for you, free of charge.

Ways To Safely Lower pH

Lowering a pH that’s too high is actually more challenging than raising it. However, there are a few safe methods you can try:

Filter Through Peat Moss

Using peat moss in your filter system or substrate is an excellent natural way of lowering the pH. Peat moss releases gallic and tannic acids into the water, dissolving bicarbonates, reducing the kH and thus the pH.

The main downside of using peat moss is that it tends to discolor the water. However, you can mitigate that problem by pre-soaking the peat moss in a bucket of water for a few days prior to adding it to your aquarium.

Add Driftwood

Driftwood works in much the same way as peat moss, releasing tannins into the water that lower the kH and pH, too.

However, the quantity of tannins contained in the wood can vary, impacting its effectiveness at lowering pH.

Add Carbon Dioxide

As mentioned above, increasing the carbon dioxide in your tank lowers the pH. So, if you add more carbon dioxide to the water, the pH will drop proportionately.

Use Chemical Additives

Acid acidic water test with ph reagent

There are plenty of chemical additives on the market these days that will lower the pH in your aquarium. However, these chemicals are not a long-term fix, and you do need to keep repeating the treatment to maintain the pH level you want.

Add Catappa Leaves

Catappa leaves are also sold as Indian Almond leaves, and you can find them in most fish and pet stores. Many aquarists like to add a few Catappa leaves to their tanks for the beneficial antibiotic qualities that the leaves offer as they decompose.

Just like driftwood and peat moss, the leaves leach tannins into the water, reducing the water hardness and, consequently, reducing the pH, too. Again, the main drawback of using Catappa leaves is that they can stain the water a yellowish-brown color.

Reverse Osmosis (RO)

Reverse Osmosis is often used by hobbyists whose tap water is heavily treated with chemicals, making it unsuitable for use in an aquarium.

RO filtration purifies the water by filtering out up to 99% of toxins and impurities such as heavy metals and pesticides through a semipermeable membrane. That helps the pH level to remain stable.

The main downside to RO is that it can work out as quite expensive. You can buy RO water from most good fish stores or invest a few hundred dollars in a Reverse Osmosis unit if you prefer. The water is crystal clear, natural, and totally safe to use, making this an excellent solution to unstable pH, especially if your tap water is very hard and unsuitable for your fish.

How To Safely Raise Your pH

If the pH in your aquarium tends to drop out of your fishes’ preferred range, there are several things that you can do to fix that.

Partial Water Changes

If the pH in your tank gradually drops over time, the easiest and most effective way to rectify that is to carry out regular partial water changes. 

To elevate the pH to that of your tap water, it’s recommended that you make several small water changes over 24 hours rather than one big one. That way, you won’t risk shocking your fish by changing the pH too quickly. 

Also, if you’re diligent in vacuuming your substrate to get rid of uneaten food and fish waste, that will help to prevent the pH from falling.

Add Substrate, Rocks, Or Shells

You can raise the water pH naturally by adding some additional substrate, shells, or rockwork to your tank. For example, a crushed coral substrate is often used by hobbyists who have African cichlids that prefer a relatively high pH. 

If you don’t want to add rocks as decorations, you could put a small bag of crushed coral or limestone in your filter. However, be vigilant if you decide to use this method, as you could end up raising the pH too much.


As we’ve mentioned earlier, if you boost the oxygen concentration in your water, you will drive down the levels of carbon dioxide, and lower CO2 means a higher pH.

So, by adding an air stone or bubbler to your tank, you’ll increase the aeration and oxygen levels in the water, which will raise the pH.

Add Baking Soda

Adding 1 teaspoon per 5 gallons of water will raise the pH in your aquarium. Dissolve the baking soda in some tank water, and then add it to your aquarium.

Baking soda on the bottle spilled on the wooden table

However, you will need to top up the baking soda periodically to maintain the pH level. You also need to be careful how much baking soda you put into the tank in one go, as that could cause a spike that might kill your fish.

Remove pH-lowering Items

It’s possible that you might have accidentally raised the pH in your aquarium by doing some of the things mentioned above in the section on how to lower pH. 

For example, did you add a new piece of wood or some Catappa leaves to your aquarium without realizing that it would affect the pH? If that’s the case, all you need to do to remedy the problem is to remove whatever it is that’s causing the pH to rise.

Chemical pH Buffers

Although there are quite a few chemical pH buffers on the market, these must be used with extreme caution, as they can cause massive spikes that could harm your fish.

Also, you’ll need to continually add more chemicals to the water to maintain the effect. Generally, if you’ve already tried everything else without success, chemical pH buffers can be used as a last resort.

In Conclusion

All aquarium fish have a preferred pH that’s close to that of their natural environment. Constant adjustment to water parameters can be deadly for your fish, so rather than trying to maintain a specific pH value, aim to keep the aquarium pH stable within your fishes’ tolerance range.

If you do need to alter the pH in your fish tank, use one of the natural, chemical-free methods we’ve suggested above, and be careful to avoid causing spikes that could be fatal for your fish.

I hope you found this guide helpful. Please put your questions in the comments box below, and don’t forget to share this article if you enjoyed it!

Alison Page has been an avid fish keeper for over 35 years and has owned many different species of freshwater tropical fish including bettas. Currently Alison has two large freshwater tanks. The first tank has two huge fancy goldfish who are almost ten years old and still looking as good as ever. In the other, she has a happy community of tiger barbs, green tiger barbs, corydoras catfish, platys, and mollies.

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