What Happens if the Alkalinity Rises Too High in an Aquarium?

by Cindy Quarters
Coral is especially sensitive to alkalinity imbalances.

Coral is especially sensitive to alkalinity imbalances.

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Fish and other forms of aquatic life do best when you keep them in an environment that’s as close to what they would get in nature as possible. This means giving them plants, places to hide and perhaps some other fish. It also means making sure that the water they live in is clean and chemically balanced so that your fish can live a long and healthy life. Serious problems can result when levels such as alkalinity go too low or too high.


Alkalinity essentially tells you how much bicarbonate is available in the water in the aquarium. It also indicates how stable the water is in terms of its pH, since water that is at least moderately alkaline does not typically experience sudden changes in pH unless the alkalinity drops. Proper alkalinity is especially vital to a reef aquarium, since corals depend on the availability of bicarbonate for survival. Aquarists should measure alkalinity on a regular basis and maintain a level that is suitable for the type of fish present, a measurement that can vary widely depending on the species.

Damage to Fish

Aquarium fish may suffer from serious problems as a result of high alkalinity in the tank. If it remains high over a prolonged period, it can dissolve the protective mucus layer that normally covers fish, leaving them vulnerable to bacterial attack and infection. Water that is too alkaline causes non-toxic ammonia to become toxic. Fish may have trouble breathing. It can also affect the fish’s fins and tails, damaging their growth and making them look ragged. Ultimately, fish in a highly alkaline environment may fail to thrive and can eventually die.

Impact on Calcium

In most cases, the residents of an aquarium can tolerate a fairly high level of alkalinity. Some enthusiasts even boost the levels deliberately in an effort to push their corals to grow faster. The main problem with a high alkalinity level is that it can interfere with the uptake of calcium by the inhabitants. In some cases where the alkalinity is too high, solids, such as calcium, may precipitate out of the water and become unavailable to the aquarium’s residents for any purpose, including growth.

Tank Problems

In addition to the problems high alkalinity can cause for fish, it may also cause problems with the tank itself and the equipment in it. When excess calcium precipitates out of the water, it ends up coating the inside of the tank, the heater, the pump and the filters. It can block your view into the aquarium and may also damage the equipment it’s covering, burning out pumps and blocking the flow of filters. Keeping the water in balance keeps the solids in solution and prevents any damage from occurring.

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