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pH in the Aquarium - Is there any "recommended pH" or "normal" value?

Ammonia test kit, resized image Testing aquarium water pH, step 1 - resized image Result of water test, pH is 7.6 - resized image Result of pH water test, pH is good - resized image

Brief Description

One of the most common questions asked by fish keepers new to the hobby is what should the pH be in my aquarium. The correct answer to this question is”there is no correct answer”. All aquariums will run at different pH levels and the same species of fish are often kept in different tanks at different pH levels, keepers browsing through the internet or reference books will see the suggested pH in the fish profiles and start panicking if their aquarium doesn’t match this.


Nowadays most specimens of fish are tank bred, they too have developed over generations of breeding to adapt to a wider range of water parameters including the pH. Most community set ups will have a pH that suits all of the species of fish that inhabit the tank, of course there are some species of fish that do require alkaline water conditions and some that will require soft, acidic water. In this case if you cannot meet their requirements it is often best to look elsewhere at different species of fish more suited to the water that you are adding to your tank.

The most important point to keep in your head about the pH is that it must be constant in the tank, if it is a little high or too low this may not affect the health of the fish as long as it doesn’t swing from one end of the scale to the other. The pH will alter slightly over a 24 hour period, it is quite common for the pH to drop at night-time and then rise again in the daytime but these swings are very small and should not affect your fish.

When first considering which fish to add to your aquarium it is a good idea to check the pH of your mains water, trying to alter the pH to suit the fish can be hard work and a constant chore to keeping your aquarium inhabitants healthy, it is much better to settle for the pH level that your water is naturally.

What is pH?

Basically the pH (power of Hydrogen) is the amount of free hydrogen ions that are present in the water, the higher the pH, the more acidic the water is. A reading of 7.0 with your test kit means that your water is neutral, there is an equal number of hydrogen ions in the water as there are hydroxide ions (HO). The more hydroxide ions that are present, the more alkaline the water ( a reading higher than 7.0). It is important to know this information as there may be time when you do need to actually alter the pH of your water by buffering it to keep it stable. Well buffered water will be more likely to give you constant test results when you are checking the pH level so what do you do if the water is not buffered enough.

Don’t panic, if the hardness of the water is very low there are ways to resolve this, the hardness and pH of the water are indirectly linked, affecting one will have an effect on the other.

Raising the pH of the water

As mentioned above the hardness of the water is the buffering agent that keeps your pH at a constant level, the hardness of the water is determined by the amount of minerals in the water. The more concentrated level of minerals, the harder the water. If this level of minerals drops then the hardness drops thus resulting in the water becoming more acidic and the pH will drop. In this instance the only way to get the pH back to the original level and hold it there is to artificially add more minerals to the water. This can be done by using commercial pH adjusters such as pH UP but these tend to be fine short term and then the pH will drop again, a much more secure method is to add a buffering agent either into your filtration system or directly into the water. Coral sand can be placed inside a mesh bag and then placed into the filter, this will slowly dissolve into the water and raise the mineral level or you can use the method that I normally use by adding Sodium Bicarbonate to the aquarium water. This should be done slowly and keep testing the water but once it has been done a few times you will soon work out exactly how much is required to buffer the water to suit the pH level that you require.

Lowering the pH of the water

This can be more difficult than raising the pH. There are also commercial products that serve this purpose but yet again they are only a short term solution. Adding peat to your filters is a good remedy, the peat will extract some of the minerals in the water thus making it more soft and acidic which will lower the pH, adding tannins or bog wood to the tank will have the same effect but on a smaller scale.

The most dramatic effect is achieved by using a water purifier, if you are using a Reverse Osmosis unit this will strip the water of most of the minerals present, in this state the water is useless for your aquarium and will not support long term life for your fish. The minerals are then replaced by re-mineralising products that will keep your pH at the required level.

If your aquarium water is buffered correctly by a level amount of mineral content, the pH level should remain constant. The pH should be checked at least once a week and always at the same time of day, remember that the pH does change over a 24 hour period.

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