What Causes High pH in Your Aquarium?

Recently, we discussed how to raise or lower your pH level if it isn’t right for your fish or coral; now we will talk about the ways your aquarium can develop a high pH level.

pH chart1 1

Check your rocks

As we have mentioned several times [raise pH article], limestone is notorious for its calcium carbonate content. Having limestone as your substrate or in your filter will increase your water’s pH level.

You can test your rocks for calcium levels by dropping a little bit of vinegar on them . If they fizz, the calcium content is high, so it will make your aquarium more alkaline. Another way to test them is to test a bucket of plain tap water for water hardness and pH levels, then add your unknown rocks to it.

Test the water again. If the levels are noticeably changed, then it’s probably best to not put those rocks in your aquarium because they may cause its pH balance to change too quickly.

Lack of beneficial bacteria

This is something you will want to deal with from the start of a new tank. The fish, plants and undigested food with produce nitrogen when, when mixed in water, can combine into nitrites or ammonia. This can cause ammonia poisoning and kill your fish even at low levels.

To reduce the risk of high levels of pH and ammonia, you should preemptively act. When you start a new tank you must establish colonies of healthy bacteria. These bacteria will break the ammonia down into nitrates and nitrites that are less harmful to your aquarium’s environment. This process is called biological filtration.

Therefore, you need to introduce bacteria to your new tank. This can be through store-bought bacteria, using fish food to encourage the growth of bacteria, or transferring gravel, toys, water or other things from an established aquarium to a new one.

After you set up your tank and introduce the bacteria, you must test the water fairly regular to see when the ammonia levels spike, then when the nitrite levels spike. When these two have returned to zero and there are now high levels of nitrates in the water, your tank has completed a cycle.

If, after you have established your tank, the pH level is still high, you should try to introduce more bacteria from a thriving aquarium of the same type or buying a product to boost your aquarium’s health. Establishing cleaning bacteria will go a long way to ensuring your water does not become too alkaline.


Related to the above, the hotter your tank is, the faster waste products break down, leading to faster production of ammonia into your water. You should monitor your tank’s water carefully for large increases in temperature. The heat will affect your water’s pH and could make your animals ill.

Low carbon dioxide levels

You tank will naturally have a higher pH at the end of a light cycle due to the simple processes of respiration. But if your tank consistently has low levels of carbon dioxide in it, your water will have a higher pH due to not being able to carry out bacterial filtration as efficiently as possible. This carbon deficiency will continue to increase until you aerate it through any of the methods mentioned in this article on How to Raise the pH Level in Your Aquarium.

Now that you have learned some of the causes behind high pH, you can work to raise or lower your tank’s pH level according to the needs of your aquarium and its inhabitants.

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