Why Aquarium Plants Turn Brown, and What to Do

The short answer

Brown on an aquarium plant is usually either a coating on the leaf or the leaf itself dying back. A dusty film that rubs off is a coating, often diatoms in a new tank. Brown old leaves on a newly bought plant are often air-grown leaves dropping. Otherwise, check nutrients, light, substrate and tankmates, and test the water.

Browning here means brown color on a leaf: a film on its surface, or brown tips, spots, patches or holes in the leaf tissue. This guide covers freshwater planted tanks. For a plant that is wasting away as a whole, our guide to why aquarium plants die covers the usual causes.

Is the brown on the leaf, or in it?

Start by rubbing a brown leaf. Canton Aquatics, a live-plant retailer, puts the test this way: “If the brown color wipes off when you rub the leaf, it’s not the plant”.

A brown coating is often algae. Aqueon, an aquarium equipment maker, says “most brown algae are a form of diatom” and that in newly set-up tanks they “will usually die off on their own after the tank cycles”. Aquasabi, an aquascaping retailer, says “These algae coat every surface in the tank, like the substrate, technical equipment, plant leaves and the decoration.” Aquarium Co-Op, a retailer that sells live plants, describes diatoms as “a dusty, flour-like substance” that “easily rubs off with an algae scrubber sponge”. Our guide to brown stuff in an aquarium covers diatom film, tannins and organic waste.

Brown water is a different problem. Aqueon says “If you’re maintaining a planted aquarium, the discoloration could be caused by natural foliage which contains tannins”, and that “tannins aren’t dangerous to your fish”. If your fish look unwell, see the warning below. Our guide to brown aquarium water covers tannins and the other causes.

If the brown stays put, the leaf tissue itself is browning.

Where the brown shows, and what it can point to

Tetra, a fish-care maker, and the sources above link each pattern to a cause. Patterns overlap, so start here, then test.

What you see	What sources link it to	Check first
Brown film that rubs off	Diatoms or debris (Aqueon, Co-Op)	Tank age; water flow
Old leaves browning on a new plant	Air-grown leaves dropping (Tetra)	Leave the plant planted; cut leaves that have melted (step 3); watch for shoots
Older leaves, yellow to brown from the tip	Too little nitrogen (Tetra: yellow to brownish; Co-Op: yellow, translucent)	Nitrate test
Pinholes rimmed brown, or black dots turning to holes	Too little potassium (Co-Op, Tetra)	What your fertilizer contains
Brown spots or soggy brown patches	Phosphate: too much (Tetra) or too little (Co-Op)	Phosphate test
Brown, holed leaves where animals graze	Snails or plecos (Tetra)	Watch the tankmates
Brown or poor growth overall	Too little light (rare), too little CO2 or compacted substrate (Tetra)	Light and substrate

New plants: leaves grown in air

Tetra says plants sold for aquariums “are usually grown above water”. Once underwater they make new leaves, and “The old leaves (‘air leaves’) turn brown and gradually drop.” Aquarium Co-Op describes newly planted crypts whose leaves are “turning yellow-brown, have large gaping holes, or are simply withering away”.

Our guide to why aquarium plants melt covers melting on its own, and mosses have a guide of their own, why aquarium moss turns brown.

Nutrients, and where the sources split on phosphate

Tetra’s guide and Aquarium Co-Op’s nutrient guide describe leaf damage from three nutrients; brown appears in Tetra’s nitrogen and phosphate lines and in Co-Op’s potassium and phosphate lines:

Nitrogen. Tetra: “A nitrogen deficiency primarily affects older leaves. These turn yellow to brownish starting from the tip.” Aquarium Co-Op describes “old leaves turning yellow and translucent, especially starting at the leaf tips”. Tetra adds that nitrogen can be checked “by using a nitrate test”. A low nitrate reading points to this shortage; none of our sources gives a number for “low”. Our guide to why aquarium plants turn yellow covers the yellow stage.
Potassium. Aquarium Co-Op describes “distinctive pinholes that are sometimes rimmed with brown or yellow”. Tetra says the first sign is “often in the form of black dots on the plants’ leaves, which then develop into holes”, and that the shortage “often results from strong lighting and the use of CO2 fertilisers”. Our guide to why aquarium plants turn black covers dark spots and leaves.
Phosphate. Here the sources point opposite ways. Tetra says “Too much phosphate often leads to brown spots developing on leaves”, while “Phosphate deficiency, on the other hand, causes poor growth and small shoots.” Aquarium Co-Op says that with too little phosphate “the older leaves are most affected and will start turning yellow with soggy brown patches”.

A phosphate test shows your reading. Tetra says an aquarium should contain “neither too much nor too little phosphate”, but neither Tetra nor Aquarium Co-Op gives a reading for either one, and they disagree on which one browns leaves. Co-Op adds that phosphate deficiency is “more uncommon, since fish foods like flakes contain phosphates”, and warns that phosphate-absorbing pads put in filters against algae can end up “starving the plants of phosphate”.

Algae is a separate question. Aqueon’s plant guide says “Nitrates should be below 10 ppm and phosphates below 0.5 ppm to prevent nuisance algae from growing on leaves.” That is a limit for algae, not for brown leaves; none of our sources gives a lower figure.

When tests show a shortage, Tetra says “you should add an appropriate fertiliser”. Aquarium Co-Op, which sells its own all-in-one fertilizer, says to try a higher dose of an all-in-one fertilizer before adding a single-nutrient supplement, because “if you’re missing nitrogen, for instance, you’re likely missing other nutrients as well”. For potassium, it adds a supplement only “If the problem still persists and you have plenty of nitrate and phosphate”. Check the product’s label for its dosing directions.

Light, CO2, substrate and tankmates

Light. Tetra says “In rare cases, brown aquarium plants can also be caused by a lack of light”. Strong light is not always better: of java fern, Aquarium Co-Op says “At the very high end range of light, you can actually burn or melt it.” Our Lighting section gathers aquarium lights.

CO2. Tetra says “Most aquariums automatically contain enough CO2 so there is no need to add more. However, if an aquarium only contains a few fish or solely contains plants, it can be useful to add CO2.” The signs it gives for a shortage are “stunted and generally poor plant growth”. Aqueon says “Most aquatic plants don’t require additional CO2 to grow, but their size, color and vibrancy are always better when it is added.”

Substrate. Tetra says “If the substrate becomes compacted, plants can no longer absorb nutrients via their roots and a nutrient deficiency can ensue.”

Tankmates. Tetra says “The leaf epidermis can be damaged by snails or plecos on the leaves”, that they also eat leaves, and that “This can then cause the leaves to turn brown and/or become perforated”.

What to do about brown leaves

Rub a brown leaf (see above)

For brown in the tissue, match the pattern in the table.

Test the water with an aquarium test kit

Aqueon says “The only safe ammonia level is zero” and “The only safe nitrite level is zero”; for a reading above zero, our guide to high ammonia in an aquarium covers the usual causes. Test nitrate too, and phosphate if spots or patches are the problem. Our roundup of freshwater aquarium test kits lists kits for ammonia, nitrite and nitrate, but no phosphate kit. Aqueon’s plant water ranges, with its caution about changing pH or hardness for fish, are in our guide to why aquarium plants die.

Cut off melting or fully brown leaves, and leave the plant planted

Aquarium Co-Op says: “If you see a leaf is clearly melting, cut it off at the base of the stem near the substrate. Rotting leaves can sometimes cause nitrogen spikes or algae growth, so it’s best to remove them unless your clean-up crew members consume the dead leaf first.” For a new crypt, the retailer says: “As long as it has healthy roots and is not moved once planted, you should see little shoots popping up within a few weeks.”

Handle a coating by the age of the tank

In a new tank, Aquasabi calls diatoms during cycling “absolutely normal, and you need not take any countermeasures”. In a mature tank, Aqueon suggests you “introduce plecostomus, otocinclus or nerite snails to control brown algae in mature aquariums”, and cautions: “Use caution when adding plecostomus to planted aquariums, as some species are known to eat broadleaf plants like Amazon swords and Anubias.” For nerites it adds: “A secure cover is required as these snails are known to climb out of the aquarium.” Weigh all of this against Tetra’s point above that snails and plecos can damage leaves.

Keep up partial water changes, after a nitrate test

Aquarium Co-Op warns that weekly water changes without a nitrate test, “while fine for fish only tanks, can lead to a lack of nitrogen”; our guide to why aquarium plants die covers the routine.

Feed the plants if tests point to a shortage, then wait

After a fertilizer change, Aquarium Co-Op says “Generally, it takes at least two to three weeks to see a difference in your plants”.

Warning

If your fish look sick or are dying, test the water now and contact an aquatic veterinarian. Plant fertilizer and plant care are not fish treatments.

Ready to buy?

Our roundup of fertilizers for aquarium plants compares the options, and our Substrate, Plants & Decor section gathers plants, substrates and planted-tank gear.

Frequently Asked Questions

Q. Should I cut brown leaves off aquarium plants?

Cut off leaves that are melting or fully brown (step 3). TFH Magazine, a fishkeeping publication, notes that plants’ dead leaves, “if not removed, will decay and produce additional wastes”.

Q. Can a brown leaf turn green again?

Canton Aquatics says of fully brown, mushy leaves: “They won’t recover and their decay fouls the water.”

Q. How long does it take for brown algae to go away?

In a new tank, Aquasabi says “As a rule, the diatoms are crowded out by green algae a few weeks later in the life of your new tank”. None of our sources gives a time for an established tank.

Q. Why is my aquarium grass turning brown?

In a Fishlore forum thread, a member told one owner that the brown part of their dwarf hairgrass was “emmersed growth” that “will die back”; the same member says hairgrass “likes a fine, nutrient-rich substrate, medium to high lighting intensity”. Our guide to why aquarium grass turns brown covers lighting, nutrients and algae.

Sources (13)

Aquarium Tales Team

Written and reviewed by the Aquarium Tales Team. See the About page for how we work.