Causes of Brown Spots on Aquatic Plants

Updated March 23, 2017

According to tropical resource site Monga Bay, aquatic plants add a number of benefits to fish tanks and water gardens. Besides adding visual interest, aquatic plants increase the quality of life for fish and help fish live a more natural life. Norton Aquatics reports that consumers can trace most problems with aquatic plants to either poisoning or nutrition issues. Brown spots on aquatic plants have three possible causes.

Excess Nitrates

One of the most likely culprits to cause brown spots on aquatic plants is excess nitrates in the water, according to the online aquatics guide If you do not change the water enough in the tank, nitrates will build up, creating small brown spots on the leaves. Eventually leaves will turn yellow and die, increasingly the likelihood of algae growth since algae can feed on dying leaves. Monga Bay reports that a moderate increase in the number of water changes will help keep nitrates at a safe level.

Nitrogen Deficiency

Plants that do not get enough nitrogen will develop small brown spots on older leaves, according to Norton Aquatics. When this happens, upper leaves will appear light green. Older, lower leaves will begin to turn yellow in a nitrogen-deficient environment. Norton Aquatics reports that once a plant's growth begins to slow or stop because of malnutrition, algae growth will increase, since the plants will not use all the nutrients in the water. Solving malnutrition problems could take as many as three weeks to see any results, reports Norton Aquatics, and the danger of over-fertilisation means handling nutrient deficiencies requires careful attention to aquatic plants.

Plant Poisoning

Brown spots will appear on poisoned aquatic plants, according to Norton Aquatics. Poisoning can occur from using untreated tap water in the tank--many municipalities put chlorine or chloramine in tap water; if you do not treat it with a dechlorinator, it will cause aquatic plants to develop brown spots. Over-fertilisation can also qualify as poisoning, since having too many nutrients in the water is toxic to aquatic plants. Norton Aquatics reports that treatments for fish diseases can poison aquatic plants. Some treatments are non-toxic to plants; if you have purchased a toxic disease treatment, either remove the plants from the treated water while the fish recover, or treat diseased fish in a separate tank.

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About the Author

Matt Browning has been writing about health, science, food and travel since 1990. His career has spanned advocacy, medical communications and public relations and his stories have won awards from the Virginia Press Association. Browning earned a Bachelor of Arts in English language and literature from the University of Virginia.