In a normal aquarium for decoration, you don’t need to change the pH value of the water unless the tap water is very bad for fish. So, a beginner who wants to keep an aquarium usually doesn’t need to worry about this parameter.

When we buy fish that are very sensitive or hard to catch, like fish caught in the wild, the situation is different. In this case, we need to make sure that the right things are in the water before we use it. Both spawning aquariums and aquariums with plants need to have the PH adjusted.

And don’t forget that fish with a bad reputation also need extra care. From what the aquarium literature says, preparing water for them may turn out to be a huge mistake that will cost the fish their health or even their lives. There have been times when brand-new discus were put into a tank with soft and sour water, which killed them. Why? Because the fish grew up in water that had a pH that was close to 7. So, if you want to buy fish that have special needs and are thought to be hard to keep, you should ask the seller or breeder what the chemical parameters of the water they have been kept in so far have been.

Changing the water’s pH always has to do with how hard or soft its carbonates are.

Water buffer system

The water buffer system is a way to keep the pH level from changing. If acid or alkali gets into the water, the buffer system gets rid of it, so the pH level doesn’t change. On the one hand, this means that the buffer system is our friend, but on the other hand, it makes it very hard to lower the pH level.


Both carbonates and bicarbonates

The main parts of the water buffer system are carbonates (CO32-) and bicarbonates (HCO3–). A test for water carbonate hardness can tell how much of them are in aquarium water. The constant supply of acids, whether they are added on purpose or made in the aquarium by biochemical processes, can reduce the amount of alkalinity in the water. The pH of the water can then change a lot, which is bad for fish’s health.

You can add special aquarium preparations or baking soda (NaHCO3) from a grocery store to make the water more alkaline. It is a source of bicarbonates, which are a part of the buffer system. The baking soda solution is a little bit alkaline, so it will make the pH in the aquarium a little bit higher. But it won’t change how hard the water is as a whole.

By adding limestone rocks and coral gravel to the aquarium, the total hardness and carbonate hardness can also be raised. The latter can be put in the filter chamber or the bedding. The buffer system will always have enough bicarbonates and carbonates because the limestone and coral gravel will break down slowly. Most of the time, these kinds of treatments are used in aquariums with fish that like alkaline water, like cichlids from Lakes Malawi and Tanganyika.


Humic substances

In “black waters,” you can see another interesting buffer system. Even though there aren’t many carbonates, humic compounds fill in for them and protect this environment from sudden changes in the pH level of the water. By passing the water through peat, the aquarium can be made to look like this. So, we lower the hardness and pH level of the water in a way that is natural and safe for living things. This is because peat has humic acids and a high sorption capacity, which means it can hold a lot of things. Ca and Mg, which affect how hard the water is, are held by peat.

Using peat to soften and acidify water is a slow process that takes place over time. This keeps water organisms from being hurt by sudden changes in these parameters. Roots, oak and sea almond leaves (ketapng), and alder cones, which are used as decorations and have humic compounds and tannins, can have the same effects as peat.


How to lower the hardness and pH level of water

Using distilled water or water from a reverse osmosis filter in the aquarium is the easiest way to lower the carbonate hardness and pH of the water. You don’t have to install a distiller at home; you can just buy distilled water. When you don’t need much, it’s easy. With more people wanting water, it’s a good idea to buy a RO filter (reverse osmosis).

Even though distillation and filtration with a RO filter work in very different ways, they both aim to get rid of things that are dissolved in water. The success of each method is different and depends on a lot of things that won’t be talked about here. In any case, we get water that doesn’t have any hardness or alkalinity from carbonates. The water will be acidic because it doesn’t have a way to get rid of carbonic acid, which is made when carbon dioxide is dissolved in it. To get the right kH/gH and pH values, the aquarium hobbyist mixes this water with tap water in the right amount or dissolves special salt mixtures in it.


Definitions of terms


During the process of distillation, the water is heated so that it will evaporate. The steam that comes out is then cooled and collected. Depending on the distiller, the water that comes out of it may have an electrolytic conductivity of about 1 S/cm (microsimmens/cm). However, the water is sterile right after distillation and doesn’t need any special equipment to stay that way, but it loses its sterility quickly when it’s stored. Since distillation is a slow process, water needs to be collected and stored for use later. If it is not well protected, volatile substances like carbon dioxide, ammonia, and other organic compounds can dissolve in it and cause secondary pollution. If the storage container is not made of an inert material, ions and plasticizers will be rinsed out of the container and into the water.


Ion exchange is used to get rid of the ions in water. Ionites, also called ion exchangers, are substances that can swap one ion for another. Ion exchange is a process that can go both ways, so ionites can be made again. The ion exchange process involves changing the H+ hydrogen ions and OH– ions in the ionite into cations, such as Ca2+, and anions, such as HCO3–. After some time, the resin will get new H+ and OH– ions. The deposit will need to be changed out for a new one or fixed up. The hardness of the water after it goes through the resins lets us know if the deposit is still working. Using a high-purity resin, almost all of the ionic compounds in the water will be removed, giving the water a maximum conductivity of about 0.055 S/cm at 25°C.

However, aquarists often use resins that swap sodium ions (Na+) for calcium and magnesium ions (Ca2+ and Mg2+) because it is easy to do so (using cooking salt). In this case, the overall hardness of the water goes down because the resin keeps the Ca2+ and Mg2+ ions. However, the alkalinity doesn’t go down because the bicarbonates (HCO3–) are still there (sodium ions enter the water). It’s important to remember this when the water is going to be used to spawn fish that need low salinity (water conductivity). Since this kind of resin doesn’t make the water less alkaline, we still won’t be able to get the pH down.


CO2 levels and the pH of the water in the aquarium

Carbon dioxide (CO2) is often used as a fertilizer for plants in aquaculture. It is meant to give carbon to aquatic plants, which is a key part of the process of photosynthesis. How well a gas mixes with water depends on how much of it is in the air. There are, however, exceptions to this rule because some gases can react with water. For example, carbon dioxide can dissolve in water more than its concentration in the air because it can react with water to make carbonic acid (H2CO3). The carbonic acid that is made is what causes the pH of the water to drop quickly or slowly, depending on the buffer system.

The right pH for plants and fish is reached by adding the right amount of CO2 when giving carbo. But keep in mind that putting CO2 into water with low carbonate hardness or alkalinity will cause the pH to drop to a level that is bad for fish. When adding CO2, measuring the pH is a very important part of keeping the aquarium running well. By measuring the pH every day, we can keep track of what’s going on in the aquarium and use what we learn to make any changes that are needed.


People sometimes ask me how rainwater can be used in aquaristics. It is soft water with a pH of about 5.6 (because CO2 dissolves in it) or lower if pollutants in the air, like sulphur and nitrogen compounds, which make the water very acidic (acid rain), dissolve in it. Low pH in rainwater also makes it easier for heavy metals that float around in the air bound to different types of dust to dissolve. Most of the time, using this kind of water is not a good idea because of how dirty the air is. If we decide to do this, we should catch the rain a few minutes after it starts to rain, when the dirtiest “first rain” is already falling and cleaning the air. This method of collecting water shouldn’t really be used in big cities. Alkaline rain can also fall, for example near a cement plant.

Products for lowering the pH of water

You can also use products with orthophosphoric acid or hydrochloric acid to lower the PH of the water. But keep in mind that the more carbonate hardness or alkalinity there is in the water, the more of a product will be needed to get the job done. Also, products with orthophosphoric acid can be a very important source of phosphorus in the aquarium, especially if we use a lot of them.

Since we are already talking about commercial products, we can’t leave out peat extracts, oak bark, or sea almond leaves that are already made. This is a faster way to get black water than filtering it through peat. These products work well as long as we remember that if we use them in water with a lot of hardness, some or all of the product will be used to bind the Ca2+ and Mg2+ ions in the aquarium, which will leave a brown deposit. In this case, the product won’t work, so before you use it, you’ll need to lower the overall hardness and carbonate hardness of the water.

Biochemical processes


The biochemical processes that happen in an aquarium cause the PH of the water and the carbonate hardness/alkalinity to go down as the aquarium ages and works well. So, the water can get the right conditions for fish on its own. A small amount of water change here and there won’t hurt them in any way. On the other hand, changing some of the aquarium water with Polish tap water, which is usually hard and alkaline, keeps pH from dropping too much due to biochemical processes and rebuilds the aquarium water’s buffer system by adding bicarbonate ions (HCO3–).


*Aquarium tests that measure how hard the water is because of carbonates actually measure something else: alkalinity. Under some conditions, the hardness of carbonate is the same as the alkalinity.


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