Planted aquariums are one of my favorites, but I am the first to admit that keeping aquatic plants is much more difficult than raising tropical fish. I am often asked, “why are my plants dying or turning yellow?” If you’ve ever wondered why your aquarium plants are turning brown or not thriving, this guide is for you!

Solving Problems With Freshwater Plants

The most common reason for yellow, brown, or black leaves on aquatic plants is poor water quality. Plants are much more sensitive to water conditions than most animals and show signs of stress when things are out of balance. In the community tanks, the plants act like canaries in the coal mine, sending out early warning signals.

Plants depend on the nutrients in their substrate and float in their water, and they depend on the lights in your aquarium to give them the ideal color spectrums for photosynthesis. When your plants aren’t thriving, that’s a sign that there’s a bigger problem with your tank. How do they troubleshoot, identify and fix the problem to get their plants back?

How To Tell If Your Plant Nutrients Are Out Of Balance

One of the first places to start looking for the problem is by examining the overall quality of the water. In addition to checking pH, water hardness, and ammonia levels, you should also examine your plants and see if you can spot symptoms that indicate an excess or lack of specific nutrients or trace elements.

macronutrients

Macronutrients are the most critical for plant health and are needed in large quantities by aquarium plants. You’ll often find these nutrients in balanced plant fertilizers, but that doesn’t mean you should just add fertilizer every time you have a problem. These are the most important macronutrients and how they affect your plants.

Coal

If you have ever taken an organic chemistry class, then you already know that without carbon (C), there is no life on planet Earth. Every living creature depends on the abundance of carbon to survive. Plants primarily use carbon from carbon dioxide (CO2), as it is an essential component of photosynthesis.

Signs of carbon deficiency

Carbon deficiency is a very common problem in planted aquariums. In fact, this is the most likely reason I have come across poor plant growth and lack of growth. Some of the signs that your tank needs more carbon include:

• Weak or stunted plant growth
• Pale or yellow leaves
• Yellow plants with dead leaves leading to dead plants.

Adjust CO2 levels in an aquarium

Since CO2 is a gas, it can be a bit more difficult to supplement in an aquarium than other macronutrients. Altering CO2 levels can also change the pH of the water, so keep an eye on it as you make adjustments. Some techniques to increase the CO2 available in your tank include:

• Use a carbon injection system or add a liquid carbon-based fertilizer to the water every day
• Turn lights on and off during the day to allow CO2 from your fish and bacteria to naturally increase levels in the water.
• Choose slow-growing plants and reduce the amount of light your tank receives to slow down your plant’s photosynthesis, allowing CO2 to build up naturally
• Reduce water currents and eliminate bubbling devices to reduce gaseous CO2 loss from your tank water.

Match

Phosphorus (P) is also a difficult macronutrient to manage, because plants can suffer when there is a deficiency, but also struggle if there is too much. Water test kits measure inorganic phosphates in your water, but cannot detect organic phosphates. Ideal levels for a planted tank are around 0.5 ppm (mg/L).

Deficient phosphate levels

Phosphates (PO4) are usually present in tap water and are added through carbon filter media, fish food, buffer solutions, and aquarium salts. Organic phosphates are produced from decaying plant materials, food scraps, and fish waste. When your phosphate levels are too low:

• Mature leaves develop disintegrating yellow patches, leading to large, ragged holes in the leaves before they fall off and die.
• If your plant’s new leaves are turning yellow and dying quickly, you probably need to check its phosphate levels and add a balanced fertilizer.

excessive phosphate levels

Higher levels of organic phosphate can increase plant growth to some extent, but also often lead to algal blooms, especially the dreaded green spot (GSA) and green water algae (GWA). This usually happens when you skip water changes and perform irregular maintenance on your tank. If there is too much phosphate in your water:

• Your plant’s leaves may turn brown or black, and the plants begin to die.
• Test your phosphate and ammonia/nitrate levels, do a 50% water change, and really vacuum your substrate to remove any decomposing materials.

Nitrogen

Nitrogen (N) is the most abundant element in our atmosphere, and both plants and your good aquatic bacteria depend on the nitrogen in your tank to grow and thrive. Aquatic forms of nitrogen include nitrate (NO3), ammonia (NH3), and ammonium (NH4). These are also essential components of the nitrogen cycle in a fish tank.

Deficient nitrate levels

An advantage of planted aquariums is that they generally do not have a problem with excessive levels of ammonia or nitrate because the plants use these nutrients before they can harm the fish. However, in densely planted tanks with few fish, you may not have enough waste to keep the cycle moving.Low nitrogen levels can cause:

• Mature plant leaves turning yellow and falling off.
• Yellow or stunted new growth
• New leaves deformed or reddish in color

Excessive levels of nitrate or ammonia

This is a less common problem in planted tanks, but if you add a lot of plants to a new tank before it’s cycled properly, you may not have enough good bacteria to break down the fish waste products into usable nitrates. Your new plants may turn yellow, brown, or black and die. I talk more about handling new tanks below.

Potassium

Potassium (K) is a critical nutrient for plant growth and one of the most common to be deficient in, as plants use it in large amounts. It’s a bit tricky to fix, as your potassium levels fluctuate and are influenced by your CO2 levels, pH, and water hardness. Most planted tanks benefit from potassium supplementation:

• When your potassium is too low, you’ll usually notice little yellow holes in your plant’s leaves.
• You may also see a yellowish discoloration along the edges of the leaves, but unlike the magnesium deficient plants below, the leaf will look normal apart from the yellow edges.

Magnesium

Magnesium (Mg) is one of the most important nutrients and helps keep your plants healthy and green. Low magnesium levels can also mimic iron deficiency, and the two go hand in hand, as a lack of magnesium reduces the amount of iron plants can absorb. Often you will have to fix the problem with a good fertilizer.

• If your magnesium is deficient, established plants turn yellow along the edges or develop yellow spots on older leaves.
• Magnesium deficiency can also resemble a potassium deficiency, but the leaf veins usually remain green even when the leaf turns yellow.

Micronutrients

Plants also use other trace elements in smaller amounts, such as iron, manganese, boron, and molybdenum. Between your substrate, fertilizers, and water changes, you’ll generally have enough of these nutrients. The most common micronutrient deficiencies are low levels of iron (Fe) and manganese (Mn).

Iron

Iron-rich substrates like Flourite and iron-containing fertilizers are a great way to prevent deficiencies and encourage lush plant growth. However, it depends on the form, as some types of iron are not soluble and therefore not available to your plants. In the early stages of iron deficiency, you’ll see symptoms like:

• New growth and leaf tips look pale, yellow, or white.
• Mature leaves turn pale yellow progressing to transparent or white, with visible green veins (chlorosis)
• In advanced cases of iron deficiency, the leaves and plants will be severely stunted and will eventually turn black as the tissues necrotic and die.

Manganese

Manganese is vital to your plant’s metabolism, and deficient plants often have problems with their photosynthesis and growth. Plants suffering from low manganese levels often have symptoms similar to those with iron deficiencies. Fortunately, most iron-rich fertilizers also contain trace amounts of manganese. Symptoms include:

• New growth looks pale or yellow
• The leaves develop yellow spots that lead to jagged holes between the leaf veins.

Additional Causes for Brown Aquatic Plants

Nutrient deficiencies aren’t the only reason aquatic plants turn brown or die. As I mentioned earlier, the general quality of the water, the lighting conditions and the substrate also play an important role in the health of your plants.

Poor aquarium maintenance

I learned the hard way that planted aquariums require more frequent water changes than fish-only tanks. When I started my first planted community I made the mistake of only doing water changes once a month, as I always had. As expected, my plants did not fare well at first.

Once I started doing more frequent water changes and adding ammonia removal media to my filtration systems, my plants started to skyrocket. I didn’t know how to diagnose specific nutrient deficiencies, so I used a balanced fertilizer after each water change. This could be all you need to do to solve a brown plant problem!

Inadequate lighting conditions

In the old days, if you wanted a thriving planted aquarium, you needed to invest in the best lighting setups and replace bulbs frequently. With modern LED aquarium lights things have become cheaper and you won’t need to change diodes as often. As long as your plants get the proper light spectrums, they will do just fine!

However, it depends on your plants. Easy-to-grow aquatic plants often do not have very strict requirements. If you are planning a densely planted tank or with an aquatic space or if you want more challenging plants, you may need a higher quality lamp to support their growth. However, excess light and nutrients can cause problems with algae!

Unsuitable substrate for planted tanks

For creating fish-only communities, your substrate is generally not a critical option to worry about. But planted aquariums rely on the substrate for nutrition and to support colonies of bacteria that break down ammonia into usable nitrates. If you choose a sandy substrate, you can prevent your plants from thriving!

Colored aquarium sand and gravel lack trace elements and nutrients to support lush plant growth. Sand also compacts easily, creating dead zones in the aquarium and substrate that lack oxygen. I recommend using a substrate designed for planted tanks and boosting the effect with a balanced fertilizer!

Plant symptom(s)	Possible causes)	remedy(s)
Leaves turning brown/black	Too much phosphorus / high phosphate levels Too much ammonia / high nitrate levels	Water changes and new maintenance schedule.Improve water conditions with better filter/filter mediaUse a fertilizer rich in iron
Mature leaves turn yellow	Potassium or magnesium deficiency	Use a balanced fertilizer Add fish or use a balanced fertilizer
Thin or weak growth		Use a balanced fertilizer
Growth stops / plants die		Use an aquarium heater to stabilize the temperature. Use an iron-rich fertilizer.
Big holes in the leaves		Water changes and new maintenance program.
Brittle leaves, yellow new growth, or yellow spots on leaves leading to holes.	Iron or manganese deficiency	Use a magnesium supplementUse an iron-rich fertilizer with trace elementsUse a balanced fertilizerCheck pH levels and correct
The leaves or the entire plant turn white or transparent.	Iron or manganese deficiency	Use an iron-rich fertilizer with trace elements.Improve lighting

Other Common Problems With Planted Aquariums

When your aquarium plants turn black or die, the first thing to consider is a nutrient deficiency, water quality issues, or a lack of sufficient light to support plant growth. But those aren’t the only reasons people fight with planted tanks!

New Tank Syndrome and Plant Problems

A very common mistake I made was adding plants to a new aquarium that had not completed the cycle. Since your plants rely on populations of aquatic bacteria to break down ammonia into nitrate, a cycling tank often has spikes and crashes in its levels, and these rapid changes can stunt and kill delicate plants.

Until you have stable populations of good aquatic bacteria, which can take about 3-6 weeks, it’s best to skip dense plantings and instead choose some hearty, fast-growing test plants like hornwort. Some clusters can consume nutrients quickly to help prevent algal blooms, and are tough enough to push through spikes.

Brown spots on plant leaves

Not all brown plant problems are related to nutrient fluctuations, at least not directly. If your plants start to develop rusty brown spots on the leaves, especially if it’s a new tank that’s still cycling, you’re likely dealing with a brown algae outbreak rather than a nutrient deficiency. If the spots rub off and feel a little gritty, it’s algae.

If you’re wondering how to clean leaves with brown spots, the truth is you may not need to bother. Brown algae usually appear in recently cycled tanks and disappear on their own once the nitrogen cycle is established. You can wait for the problem to be fixed, do additional water changes to reduce nutrient levels, or get an otocinclus that feeds on algae!

conclusion

It is easier to prevent your aquarium plants from turning yellow, brown or black than it is to bring them back from the dead. If you’re diligent about maintaining water quality, choosing the right growing medium and lighting conditions for your type of plants, and regularly applying a balanced fertilizer, your plants will probably stay green!

When and if plant problems arise in your tank:

• Fix the problem by examining your plants and test your water for the most likely suspects
• Do a water change or two and make sure your filtration system and lights are in good working order
• Use the right kind of fertilizer (if necessary) and change your maintenance schedule to get things back on track.