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What Causes the pH to Drop in Freshwater Aquariums? Answered

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Keeping the tank parameters in check is essential for the health of any aquarium fish and plants. One of these tank parameters to maintain is the pH.

A decreasing pH in your aquarium is usually caused by uncleanliness in the tank. This includes excess fish waste, leftover food remnants, and decayed matter. If the tank is not properly maintained this waste becomes nitrate (acidic) and brings the pH down. 

Read on to learn more about what to do if your pH becomes low and how to prevent it in the first place!

pH Parameters

Stability in your pH levels is key to maintaining water quality in your freshwater aquarium. Water pH is measured on a scale from 0 to 14 range.

The perfect pH level is right in the middle at 7.0, which is considered neutral. Maintaining a 7.0 pH balance is key. Below a 7.0 pH level, your water is acidic.

On the other hand, a pH level above 7.0 is alkaline. However, different fish can handle different levels of pH, what might be habitable to one kind may not be for another kind of fish. 

How to Test the pH in your Tank

How to test the pH of tap water

Many will test the pH of their tap water right away, but this is not an accurate way to test the “true” or actual level of your pH. 

The most accurate way to test your water’s pH level is to pour your tap into a clean bucket, with an air stone to oxygenate the bucket water.

From this point, you are actually going to set this bucket aside and let it sit for 24 hours. Only after 24 hours will you test the pH level of the water. 

To further test your water, leave it another 24 hours, once the bucket has been sitting for 48 hours, test the pH level again. 

Measuring the pH level at 24 hours, and then again at 48 hours, show you how the pH level changes as it gets more oxygenated (which will show how it will change in your tank), as well as giving you the most accurate reading when testing the pH level.

The reason that this is the best way to measure your pH is that as you leave the water out, and as it gets oxygenated, the carbon dioxide will cause the pH level to drop.

Aquarium Test Strip

pH test aquarium

The best way to maintain a healthy pH level in your tank is to purchase an aquarium test kit, these are available at a local pet store or online.

There are different kinds of test kits, one you take a water sample and drop it in a test solution, other you can dip a test strip into your tank water.

It is possible to get an incorrect reading from the test of your water. If you get a test result that is too high or too low there are a few things that you can do before a retest to ensure you are getting an accurate reading. 

First, make sure that your test kit is not expired. Most test kits have an expiration (shelf life), typically 6 months, if your test kit is past the shelf life, this could be the reason for an inaccurate test result and a new kit would need to be purchased to ensure an accurate reading.

Also, ensure that you are reading and following the directions on your test kit.

For example, in a test kit that uses solutions, if the directions state to shake a bottle for a full minute and you did have that time then that could also cause an inaccurate reading on your test.

If you have ensured that your test kit is within shelf life and you are following the directions as stated on the test kit packaging, you can take a sample of your tank water and bring it to your local pet store to have it tested. 

How to Raise the pH in Your Aquarium

It is typically best to acclimate your fish to the pH level of your water than to raise or lower the pH level for your fish. However, if you prefer to adjust pH levels according to your fish there are a few things that you can do to raise the pH level of your water, some of them include: 

Partial Water Changes

A partial water change in your tank is the best way to raise the pH level back into your water. This is because the more time that passes the pH level will drop as it gets more oxygenated, as mentioned above. 

This is why doing regular partial water changes is important in maintaining your freshwater aquarium. 

Doing a partial water change every few weeks, at a pace of 10- 20% of the water helps maintain a healthy pH level.

If you do not do regular consistent water changes, then when your pH level drops to low you can do larger amounts.

However, if you need to do larger amounts of a water change to bring the pH level back up it is best to space out larger changes by at least 24 hours to not stress out your fish. 

Baking Soda

Adding baking soda to your tank water is another way to raise your pH levels. However, this is something that should be done carefully to not kill your fish. Baking soda raises the pH as well as the kH levels of the water. 

This method is not a one-time deal and if you use baking soda to raise the pH during water changes then it is something that you will have to do regularly during water changes.

Also, this should be done slowly so your fish do not go into shock. It is best to put a day or two in between doses until desired pH level is reached. Doing it slowly ensures you will not put your fish into shock. 

The baking soda method has a general rule of 1 teaspoon for every 5 gallons of water. For example, if you have 15 gallons of water then you would add 3 teaspoons of baking soda.

This is done by dissolving baking soda in some water first and then adding that baking soda water into the tank. 

However keep in mind that if you have a 15-gallon tank, but you have decorations, rocks, and substrate then your tank does not truly hold 15 gallons of water. 

Rocks and Substrate 

There are some rocks or substrates that you can add to your tank that have the effect of raising the pH levels in your tank, such as crushed coral and aragonite sand, for example, that are both made of calcium carbonate.

However, with this method, you will need to monitor your tank pH levels after introducing these rocks or substrates to ensure that they are not making your pH levels go too high


Aeration is a great way to raise the pH levels in your tank. Similar to when testing your tap water pH levels with the bucket as mentioned above. 

Aeration increases the oxygen level in your tank water which brings down the carbon dioxide in the water. The lower your carbon dioxide the higher the pH level.

A great way to add oxygen to your tank is by adding a bubbler. This will constantly add oxygen to the water. You can also increase oxygen by adding live plants to your aquarium.

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