pH value remains below 6.5 even after a water change


I have a 500 liter South America aquarium. Generally, everything is OK, but the pH value goes down with every water change (100 - 125 liters every fortnight). The pH value is controlled by a seramic. I use conditioned R/O water for water changes, whose pH value is between 6.8 and 7.2.

just a remark: The pH value of R/O water is not really important, it can vary strongly as it is not buffered. Furthermore, conventional pH electrodes as they are common (and completely sufficient) in the aquarium hobby are only partially suitable for water with extremly low conductivity.

According to the data supplied by the water provider, our tap water has a pH value of approximately 7.3. Whenever I carry out a water change, the pH value sinks to 6.25 - 6.3, according to the seramic. I already reduced the CO2 addition to 15 bubbles per minutebut the the pH value does not rise above 6.4 - 6.5 between the two water changes. I already replaced the measuring probe - to no avail. Correct values are displayed when I immerse both mesuring electrodes in the test solution. What can be the cause for the constantly sinking pH value? 

Besides CO2 (including the amounts produced by the fish and micro organisms), acids formed in the are a likely cause - this is an entirely normal process; nitrification finally leads to the formation of nictric acid. Possibly present humic acids, e.g. from peat filtration, buffer the water in the slightly acidic range and accordingly can also contribute to an acidic pH value.

And, above all: How much may the value sink without plants and fish being harmed?

I do not think there will be any problems until pH 6.0, the mentioned fish can at least temporarily live in the pH range between 5.5 - 6 without being harmed.


Best regards

sera GmbH

Dr. Bodo Schnell


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