How to adjust the pH of the aquarium water

In an average decorative aquarium you do not need to regulate the pH value of the water, unless the tap water we have is extremely unsuitable for fish. Therefore, a beginner aquarium hobbyist usually does not have to worry about this parameter.

When to adjust the pH of the aquarium water?

The situation is different when we purchase particularly delicate and demanding species of fish, including wild-caught fish. In this case, we need to prepare water with the right parameters in advance. PH correction is also required in spawning and plant aquariums.

And remember that extra care is also needed for fish with a problematic reputation. It may turn out that preparing water for them, according to the aquarium literature, will be a huge mistake that fish will overpay with their health and even their lives. We know cases when newly purchased discus were put into a tank with soft and sour water, which turned out to be deadly for them. Why? Because the fish grew up in water with a pH close to neutral. Therefore, when buying fish with specific requirements, which are considered problematic, it is necessary to find out from the seller or breeder what were the chemical parameters of water ithey have been kept in so far.

How does water buffer system work?

The water buffer system is a system that prevents pH fluctuations. If acid or alkali appears in the water, the buffer system neutralizes it, preventing the water pH level from changing. On the one hand, the buffer system is therefore our ally, but on the other hand it is a serious obstacle when we want to lower the pH level.

Carbonates and bicarbonates

The water buffer system consists mainly of carbonates (CO32-) and bicarbonates (HCO3). Their concentration in aquarium water is measured using a test for water carbonate hardness . The alkalinity of the aquarium water can be depleted, used up as a result of the continuous supply of acids, whether added on purpose or formed in the aquarium as a result of biochemical processes. The pH of the water can then fluctuate significantly, which affects the health of fish.

Adjusting pH of the water is always related to its carbonate hardness/alkalinity*

To increase the alkalinity of the water you can add special aquarium preparations or baking soda (NaHCO3), purchased from a grocery store. It is a source of bicarbonates, which are part of the buffer system. The baking soda solution is slightly alkaline, which will increase the pH in the aquarium a bit. However, it will not increase the overall hardness of the water.

The total hardness and carbonate hardness can also be increased by placing limestone rocks and coral gravel in the aquarium. The latter can be placed in the bedding or filter chamber. Slowly dissolving limestone and coral gravel will be a constant source of supply of bicarbonates and carbonates for the buffer system. Such treatments are most often used in aquariums with fish that prefer alkaline water, e.g. cichlids from Malawi and Tanganyika lakes.

Humic compounds

Another interesting buffer system can be observed in „black waters”. Although there are not many carbonates, their role is taken over by humic compounds, effectively protecting this environment from sudden fluctuations in the water pH level. Such conditions can be recreated in the aquarium by filtering the water through peat. This way we reduce water hardness and its pH level in a way which is natural and safe for living organisms. Such action of peat is caused by humic acids and the high ability of peat to bind elements (so-called sorption capacity), thanks to which Ca and Mg, which shape the water hardness, are bound by peat.

The process of water softening and acidification using peat takes place gradually over time, which protects water organisms from sudden changes of these parameters. Similar effects to peat can be obtained using the roots, oak and sea almond leaves (ketapng) and alder cones, which contain humic compounds and tannins, used as decorations.

How to lower water pH level and carbonate hardness

The easiest wat to reduce the carbonate hardness of water and its pH is to use distilled water or water obtained from reverse osmosis filters in the aquarium. You don’t have to install a distiller at home, just buy distilled water. It’s convenient when you don’t need much. With higher demand for water it is worth investing in a RO filter (reverse osmosis).

Although distillation and filtration through a RO filter work on completely different principles, their common aim is to remove dissolved substances from water. The effectiveness of both methods is different and depends on many factors, not to be discussed here. In any case, we obtain water that is devoid of general hardness and carbonate hardness/alkalinity. The water will be acidic because without a buffer system, it is not able to neutralize carbonic acid, which is formed by dissolving carbon dioxide in it. Such water is mixed in an appropriate proportion (worked out by the aquarium hobbyist) with tap water or special salt mixtures are dissolved in it to obtain appropriate kH/gH and pH values.

How does a reverse osmosis (RO) filter work?

The phenomenon of reverse osmosis uses a semi-permeable membrane that allows water molecules to pass through, trapping impurities that are too large to pass. Reverse osmosis makes it possible to remove particles as small as bacteria, viruses and individual ions from the water. The holes in an osmotic membrane are about 0.0005 microns in size, while bacteria range from 0.2 to 1.0 microns and viruses from 0.02 microns to 0.4 microns.

RO filter allows to obtain soft water which is free of dissolved salts including those responsible for the formation of total and carbonate hardness. RO filters also remove organic compounds, including those formed by disinfecting tap water with chlorine compounds, as well as heavy metals.

What is water distillation?

In the distillation process the water is heated to evaporate. The resulting steam is condensed and collected. Depending on the distiller, the water obtained from it may have an electrolytic conductivity of about 1 μS/cm (microsimmens/cm), but immediately after distillation it is sterile without any special equipment and storage it quickly loses this sterility. Distillation is a slow process which makes it necessary to collect and store water for later use. If it is not well protected, volatile substances (carbon dioxide, ammonia and various organic compounds) dissolve in it, causing secondary pollution. If the storage container is not made of inert material, ions and plasticizers will be rinsed out of the container and contaminate the water.

What is water deionization?

The water deionization process uses ion exchange. Substances that have the ability to exchange one ion for another are called ionites or ion exchangers. Ion exchange is a reversible process, which means that ionites can be regenerated. The ion exchange process consists in the exchange of H+ hydrogen and OH ions contained in the ionite into cations e.g. Ca2+ and anions e.g. HCO3. After some time all H+ and OH ions in the resin will be replaced. The deposit will need to be replaced with a new one or reconditioned. We check whether the deposit is still working by testing the hardness of the water after passing through the resins. Using a high-purity resin, virtually all ionic compounds will be removed from the water, giving a maximum conductivity of approx. 0.055 μS/cm (at 25°C).

However due to the ease of regeneration (using cooking salt), aquarists often use resins that exchange sodium ions (Na+) for Ca2+ and Mg2+. In such a situation, the overall hardness of the water decreases (the resin retains Ca2+ and Mg2+ ions), but its alkalinity does not decrease (bicarbonates (HCO3) are left) just like the conductivity (sodium ions enter the water). One needs to keep it in mind when the water is to be used in spawning aquariums with fish requiring low salinity (water conductivity). Since such a resin does not reduce the alkalinity of the water, we will still have problems with lowering its pH.

CO2 dosage and pH of the aquarium water

In plant aquaristics carbon dioxide (CO2) is very often used as a fertilizer. It is intended to provide aquatic plants with carbon, an essential element in the photosynthesis process. The solubility of a gas in water depends on its concentration in atmospheric air. There are, however, exceptions to this rule, due to the fact that some gases can react with water, e.g. carbon dioxide, which in reaction with water forms carbonic acid (H2CO3), can dissolve in water more than its concentration in the atmosphere. The resulting carbonic acid is responsible for a faster or slower (depending on the buffer system) drop in water pH.

The dosing of CO2 when supplying carbo is also used to achieve the right water pH for plants and fish. However, remember that dosing CO2 to water with poor buffering properties (low carbonate hardness/alkalinity) will cause the pH to drop to a level that is dangerous to fish. When fertilizing CO2, pH measurement is a very important element of controlling the aquarium functioning. On the basis of pH measurements in the daily cycle, we can follow what is happening in the aquarium and use the conclusions to make necessary changes in the tank.

Water treatment products for lowering pH

PH of the water can also be reduced by using special products with orthophosphoric or hydrochloric acid e.g. Aquacid pH Minus. It should be remembered, however, that the greater the carbonate hardness/alkalinity of the water, the more of a products will be needed to achieve the intended purpose. Additionally, in case of products with orthophosphoric acid, they can become an important source of phosphorus in the aquarium, especially if we apply significant amounts.

As we are already talking about commercial products, it is impossible not to mention ready-made peat extracts, oak bark or sea almond leaves. This is a faster method than peat filtration to obtain black water. These products are good and effective provided that we remember that using them in water of considerable hardness will result in the use of some or all of the product to bind the Ca2+ and Mg2+ ions present in the aquarium, which will result in a brown deposit. Of course, in such a case, the product will not be effective, so for it to work you need to reduce the overall hardness and carbonate hardness of the water before using it.

Biochemical processes that lower pH of the aquarium water

PH of the aquarium water of the water and carbonate hardness/alkalinity in a mature, properly functioning aquarium decreases due to the biochemical processes that take place in it. This way water can naturally gain the right parameters for fish. Occasional partial water changes will not harm them in any way. On the contrary, partial changes of aquarium water for tap water, which is most often hard and alkaline in Poland, prevents serious pH drops as a result of biochemical processes and rebuilds the aquarium water buffer system by providing bicarbonate ions (HCO3).

Rainwater in aquarium?

Sometimes people ask me about the use of rainwater in aquaristics. It is soft water, with a pH of approx. 5.6 (due to dissolution of CO2) or lower when the pollutants present in the atmosphere, such as sulphur and nitrogen compounds, which acidify it strongly (acid rain), are dissolved in the water. Low pH of the rainwater also promotes the dissolution of heavy metals, which float in the atmosphere bound in different dusts. Generally, due to the high level of atmospheric pollution, the use of such water is not a good solution. If we decide to do so, we should catch rain a few minutes after the start of the rainfall, when the dirtiest, „first rain” purifying the atmosphere already falls. In large urban centres this method of water collection should rather be abandoned. Rain can also be alkaline, e.g. in the area of a cement plant.

How to raise pH?

I have already discussed various ways of lowering the pH of aquarium water. But there are occasions when the aquarist needs to obtain alkaline water. When you already have hard tap water with neutral or slightly alkaline pH, all you need to do is add products such as Aqua-Alkal pH Plus. You can also use special aquarium salts dedicated to aquariums with fish from Lakes Malawi and Tanganyika.

The use of baking soda (NaHCO3) is also popular. It is a source of bicarbonates, which are part of the buffer system. The baking soda solution is slightly alkaline, which will increase the pH in the aquarium a bit. However, it will not increase the overall hardness of the water. The total hardness and carbonate hardness can also be increased by placing limestone rocks and coral gravel in the aquarium. The latter can be placed in the bedding or filter chamber. Slowly dissolving limestone and coral gravel will be a constant source of supply of bicarbonates and carbonates for the buffer system. Such treatments are most often used in aquariums with fish that prefer alkaline water, e.g. cichlids from Malawi and Tanganyika lakes.

*Aquarium tests measuring the carbonate hardness of water, in fact they measure another parameter – alkalinity. Under certain conditions the carbonate hardness is equal to alkalinity.

Ph.D.Eng Aleksandra Kwaśniak-Płacheta


Kowal A.L., Świderska-Bróż M., (1997), Oczyszczanie wody, PWN, Warszawa-Wrocław;

Lampert W., Sommer U., (1996), Ekologia wód śródlądowych, PWN, Warszawa;

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