How to Reduce the Alkalinity of Aquarium Water

How to Reduce the Alkalinity of Aquarium Water

Is the alkalinity of your aquarium water high, and you are in doubt about what it could be and how to lower it? This article will explain why this is happening in your aquarium!

       High alkalinity can be excellent for fish that tend to naturally live in hard, alkaline waters, such as African cichlids from Lake Tanganyika and Malawi. For them, a pH between 8 and 8.5 can be excellent, so the first thing to do is test water with a test kit.

       In an aquarium, it is practically impossible for the alkalinity to rise without an alkalizing element, such as limestone and shells. The only way for the pH to increase in the aquarium is in the presence of a resin or material that does cation exchange, such as some special resins.

       Generally, the cause can be an alkaline substrate or decor that contains shells or limestone.

How do I know if my water is alkaline?

       First of all, test the pH value of your tap water and then test that of your aquarium water to see if the values are the same. If your tap water has a high pH, say 8, and you keep fish that prefer neutral (7) or low (6) pH, you will need to stop using that water or use products that lower and buffer the pH to an optimal value, for the fish.

       The pH can change throughout the day, so test at various times of the day. The ideal is to test in the morning and night to see if there is any change. Living plants produce CO2 at night, which lowers the pH, so it will be at its lowest level just before the lights come on in the morning. As they photosynthesize, they produce oxygen, which raises the pH throughout the day, typically higher just before the lights go out at night.

       However, if your tap water has a low or neutral pH, but your aquarium has a high pH, something inside your tank is causing it to rise. This is much more common than we think!

Reducing the alkalinity in the tank water

       As said before, the alkalinity of the aquarium will hardly increase if there is no alkalizing present in it. Still, many hobbyists need to lower its alkalinity after performing a water change. Which is often difficult due to the supply water’s high alkalinity and buffering capacity.

       Listed below are the easiest and safest methods to lower the alkalinity of your aquarium water.

Using CO2

       CO2 in contact with water produces carbonic acid, which lowers the pH. CO2 is widely used in planted aquariums to provide carbon to plants.

       By constantly using CO2, you create a buffering effect with sodium bicarbonate.

       The concentration of CO2 to be maintained depends a lot on the buffering capacity of the system, so it is up to the aquarist to find the necessary attention.

Wood, peat, and other plant-origin materials

       Whether wood, peat, coconut fiber, almond leaf, or other plant material, they acidify the water due to the organic acids that come from their decomposition.

       Each driftwood, almond leaf, coconut fiber, and the like have different organic compositions, even comparing the same species, so there is no formula for the amount of each one to use. It is up to the aquarist to see how much you need to put in to control the low pH.

Acids and Commercial Products

       The simplest method of lowering the pH is adding an acidic element to the aquarium either by directly using acids or commercial alkalinity-controlling products.

       Many organic and inorganic acids can be used, such as acetic acid (found in vinegar), citric acid (found in fruit), and phosphoric acid.

       Be very careful when handling acids!

       Each acid has an ionizing capacity and a conjugate base that needs to be considered. Phosphoric acid, for example, releases phosphate at the end of its hydrolysis reactions, which can be good or bad depending on the aquarium.

       Apply a few drops of the acid until you reach the desired pH – as simple as that. It is practically impossible to measure the amount of acid to use in a given aquarium; it depends on many unknown factors and complex calculations. The best thing to do is to dose a little, analyze the variation, and repeat until reaching the appropriate pH.

       You can easily find specialized products in any aquarium store; always follow the instructions on the package exactly.

Use of RO/DI/Rain water

       These types of water are considered pure, with a pH of 7, and free of minerals and other elements.

       Use this water to mix with your water from the supply network, thus reaching the ideal parameters for your tank.

Rocks and decoration

       When purchasing a decoration or substrate, make sure it is inert. Inert substrates will not change the pH. Also, avoid rocks intended for marine aquariums, as they will increase the pH of the water too much.

       If you have any ornaments that look like they are made of concrete or any rock that is not inert, remove them and do several significant water changes. That should already make a difference!


       As we can see, it is easy to find out why the aquarium water has a high alkalinity. It can be a limestone rock, an alkaline substrate, and decorations, especially tap water that is already coming with a high pH.

       Try testing the water to determine where this increased value comes from and use the best technique to reduce its alkalinity. Once you identify this problem, take some of the steps found in this guide to resolve it, as your fish need to live in ideal conditions for them!


        Colt, J., & Kroeger, E. (2013). Impact of aeration and alkalinity on the water quality and product quality of transported tilapia—a simulation study. Aquacultural engineering, 55, 46-58.

        de Holanda Cavalcante, D., Caldini, N. N., da Silva, J. L. S., dos Santos Lima, F. R., & do Carmo, M. V. (2014). Imbalances in the hardness/alkalinity ratio of water and Nile tilapia’s growth performance. Acta Scientiarum. Technology, 36(1), 49-54.

        Masser, M. P., Rakocy, J., & Losordo, T. M. (1999). Recirculating aquaculture tank production systems. Management of recirculating systems. SRAC Publication, 452.

        Pungrasmi, W., Phinitthanaphak, P., & Powtongsook, S. (2016). Nitrogen removal from a recirculating aquaculture system using a pumice bottom substrate nitrification-denitrification tank.Ecological engineering, 95, 357-363.

        Summerfelt, S. T., Zühlke, A., Kolarevic, J., Reiten, B. K. M., Selset, R., Gutierrez, X., & Terjesen, B. F. (2015). Effects of alkalinity on ammonia removal, carbon dioxide stripping, and system pH in semi-commercial scale water recirculating aquaculture systems operated with moving bed bioreactors. Aquacultural Engineering, 65, 46-54.         Young, R. T. (1923). Resistance of fish to salts and alkalinity. American Journal of Physiology-Legacy Content, 63(2),

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