How to Reduce the Alkalinity of Aquarium Water

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Clown fish in small aquarium.
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To reduce the alkalinity of an aquarium's water, you must remove some of the dissolved minerals. Although alkalinity is often used as a synonym for "basic," it is important to understand that the term properly refers to the buffering capacity of water. As several popular aquarium fish, such as South American cichlids, require soft, acidic water with low alkalinity to thrive, it is important to know how to reduce your tank's alkalinity when necessary.

Interconnected Variables

The pH, alkalinity and relative hardness of the water are all interrelated. According to Aquatropics Aquarium Center, proper pH levels allow the chemical reactions taking place in the water to proceed as they should, while excessively high or low pH levels impair these processes. The relative hardness of the water refers to the amount of dissolved minerals in the water. These dissolved minerals, including carbonate, bicarbonate and borate, prevent drastic changes, or buffer, the water's pH. The water's capacity for resisting pH change is called its alkalinity.

Associated Adjustments

While slightly lowering the pH of the aquarium water is a relatively simple process, it is difficult to drop the pH drastically or maintain low pH values without suitably low alkalinity. Even if you are able to lower the pH, the results will only be temporary, as the buffering capacity of the water will cause the pH to rebound quickly, causing dangerous pH fluctuations. To lower the pH of your tank enough to keep your cichlids or angelfish happy, you must first reduce the hardness, and therefore the alkalinity, of the water.


Reverse Osmosis

The most effective way to remove dissolved minerals from an aquarium's water is by using a reverse osmosis filter. Reverse osmosis filters force water through a semipermeable membrane that traps and removes solids from the water. In addition to removing dissolved minerals from water, reverse osmosis filters remove some toxic substances, including chlorine, phosphate and heavy metals.

Don’t Go Too Low

Be careful that you do not allow the pH to fall too low after reducing the alkalinity. With a reduced capacity for buffering acids and bases, the tank will now be more susceptible to fluctuating pH levels. Shell-based substrates, biological processes and tannins that leach out of any driftwood in the tank are all capable of raising or lowering the pH outside of tolerable levels. Monitor your pH levels regularly with an aquarium test kit. This will help you to catch changing pH levels quickly, before the water becomes dangerous for your fish.


Avoiding the Problem

One of the easiest ways to avoid problems with high alkalinity tank water is to begin with soft, low-alkalinity water. The alkalinity of municipal water varies from region to region, so always test your tap water before using it to fill your aquarium. If your tap water is too hard, consider installing a reverse osmosis filter on the tap or using another source of water for your fish. You can purchase pretreated water from fish supply stores or distilled water from the grocery store; alternatively, you can collect and use clean rainwater, which is usually very soft.