Why Are My Aquarium Plants Turning Brown?

Aquarium plants often turn brown without proper care. 

While adding some greenery usually helps to enhance water quality, and an unhealthy plant can do more harm than good to your tank.

Here, we’re going to go over why underwater plants turn brown and what you can do to prevent it from happening. 

Nutrient Imbalances

The most common cause of brown spots in aquarium plants is a nutrient imbalance in the tank.

Sometimes, there can be an issue with the water composition, especially if you don’t treat and condition it first.

If you notice your aquarium plants turning brown, one of the first things you should do is assess water quality

The pH should be between around 6.5 and 7.8, depending on what types of plants you include in your setup. 

Ammonia levels should be at or close to zero. Additionally, test strips shouldn’t register water as overly hard.

More often, aquarium plants begin to turn brown because they lack essential nutrients or trace elements that they need to survive. 

Aquarium owners must be diligent about macro and micronutrient levels in their tank if they want to see healthy, green growth.


Carbon is essential to growth across all walks of life. While fish obtain carbon from their food, plants get it from carbon dioxide dissolved in the water around them.

Carbon deficiency is one of the most widespread causes of browning in aquarium plants. 

If you suspect that your plants are languishing due to a carbon deficiency, you can combat it by adding more CO2 to the water

The easiest way to increase CO2 levels is by using a commercial carbon injection system or treating your tank with a carbon-based liquid fertilizer. 

Just be sure to monitor pH carefully as you treat your water to prevent it from becoming overly acidic.


Phosphorus is vital to the photosynthetic process and plant growth. Without enough unusable phosphates available, plants will yellow, wither, and die. 

However, high phosphate levels can also cause issues. Anything more than 0.5 ppm may cause aquarium plants to turn brown or black before dying. 

Organic phosphates occur naturally in tanks as plant matter, old food, and waste decay in a tank. Without proper cleaning, levels can rise dangerously high for both fish and plants.

The best way to manage phosphate levels is to clean your tank and change the water regularly. 

If you still see high phosphate levels after or between cleanings, you can find phosphate-absorbing substrates. 

With chemical absorbers, you must keep an eye on tank phosphate levels to ensure they don’t dip too low. 


The nitrogen cycle is an integral part of any tank’s ecosystem, especially when live plants are involved. 

Nitrogen is one of the primary components of chlorophyll cells, which plants need to soak up light for photosynthesis. 

While plants can’t use nitrogen in its pure form, they can take up nitrates from the surrounding water. 

These nitrates come from nitrifying bacteria in the tank substrate. Colonies convert ammonia to a less dangerous and more usable form. 

Without enough nitrogen, plants can’t produce energy. Their leaves turn yellow or brown, and new growth is withered or stunted.

On the other hand, high nitrogen levels are more dangerous for aquarium fish populations than for plants. 

The best way to keep nitrogen levels in balance is by changing tank water out weekly. 

You should also watch how much you feed your fish. Uneaten food that settles in the substrate can break down and raise nitrogen levels in the surrounding water.

Also read: How to Start Nitrogen Cycle in an Aquarium


Magnesium is another core component of chlorophyll. It keeps plants looking healthy and green, but a deficiency can result in brown or yellow growth.

If your plants aren’t getting enough magnesium, you can add a commercial supplement or substrate designed to help boost overall levels in your tank. 

It’s not uncommon for tank owners to mistake a magnesium deficiency for an iron deficiency. Fortunately, the two often go hand-in-hand, making it easier to perform a one-time treatment.


Plants need to respire to absorb carbon from surrounding CO2 gas.

They use microscopic holes called stomata to exchange CO2 in the water for waste byproducts such as oxygen and water vapor.

Plants control these stomata with ion-exchange channels that use potassium. 

Without enough potassium present in the water, aquatic plants can’t take up the CO2 they need for healthy growth. As a result, the edges of leaves are often yellow or turn brown over time.

If you suspect your plants are potassium deficient, you can add a supplement to your tank to raise levels in the water.

Keep in mind that potassium levels are prone to fluctuation. They’re heavily dependent on factors such as water acidity, current CO2 levels, and more.

You should keep a careful eye on potassium ppm and perform regular water tests to keep your aquarium plants in top condition. 


In addition to essential macronutrients, plants need trace amounts of micronutrients to stay healthy. 

While it can be harder to detect micronutrient levels in the water, signs of deficiencies are usually apparent in plants. 


Iron deficiency is a common issue in aquarium plants, but it can take some time to notice. Symptoms often progress slowly.

Early signs of iron deficiency include brown or yellow new growth. Mature leaves also turn pale, often displaying mild to severe chlorosis

After weeks or months of low iron levels, you may notice plants growing more slowly. New growth may be stunted or yellowed. Old leaves often turn black or brown as the tissue dies.

You can increase iron levels in your tank by including a natural, iron-rich substrate such as fluorite in your tank setup. 

You can also find commercial aquarium fertilizers designed to boost iron levels in the water. 


A manganese deficiency can affect an aquarium plant’s metabolism, impeding photosynthesis and stunting growth. 

Plants suffering from low manganese levels will often have yellow or brown spots. Fresh growth, if it occurs, will appear pale green or light yellow.

Most commercial fertilizers that boost iron levels also add more manganese to the water. 

You can also find water supplements containing trace amounts of manganese along with other essential nutrients. 

Unclean Conditions

Plants need plenty of fresh, clean water surrounding them to thrive. 

A dirty or poorly maintained tank can cause plant health to deteriorate quickly due to fluctuating pH and nutrient levels. 

With too much sediment of algae in the water, plants may also have trouble getting enough light. These conditions may lead to brown spots on your greenery.

While plants do help to filter the water, this doesn’t mean that aquarium owners can neglect their regular maintenance schedule. 

In fact, tanks with plants may require more frequent cleaning than those without. It’s best to change out water at least once per week for optimal plant health. 

Installing a proper filtration system can also help to ensure that aquarium plants have access to clean water. 

It’s essential to get a filter that removes dangerous chemicals and components such as ammonia. Doing this keeps your tank’s ecosystem healthy and balanced between cleanings. 

Poor Lighting

Just like the plants in your garden, live aquarium growth needs plenty of light to survive. However, there’s a delicate balance that tank owners must reach. 

With too much light, you can end up encouraging algal growth that can obscure your water and suffocate your fish. 

Too little light, and you may see stunted growth and brown spots on your plants. 

Placing your tank beside a window gives plants access to natural light each day, which can save you money and energy on artificial lighting. 

However, keep in mind that direct sunlight may be too harsh for some aquatic plants and can lead to algae blooms.

Incandescent lights used to be a popular choice of artificial lighting, but these days, most owners opt for LED bulbs instead. 

Not only are incandescents inefficient and expensive, but they can heat aquarium water to dangerous temperatures. 

LED bulbs are cool, energy-efficient, and widely available at just about any hardware or DIY supplier. 

What’s more, you can find lights offering a variety of different spectrums. You can develop a targeted lighting setup for the various plants in your aquarium to achieve maximum impact.

Also read: Should You Leave the Aquarium Light on Overnight?

Inappropriate Substrate

The substrate at the bottom of a tank isn’t usually a top concern for most fish owners.

Many choose a material such as glass beads or colored gravel based on aesthetics. 

However, with live plants, you need to pay close attention to the substrate you use. 

Plants thrive best with a substrate that contains trace minerals and nutrients. They need something that can support the bacterial colonies breaking down ammonia in the tank. 

Without usable nitrates, plants will turn brown, wither, and eventually, die. 

Dyed, artificial gravel often makes an inappropriate substrate for tanks with plants. Unlike natural gravel, it doesn’t support healthy bacterial growth. 

Sand also makes a poor choice, as it compacts down to prevent access to oxygen and other nutrients. 

You can find porous substrates specially designed to support plant life in a healthy aquarium setup. 

Often, these contain essential nutrients and minerals to help both plants and beneficial bacteria to thrive. 

Also read: How Much Gravel Does An Aquarium Need?

In Conclusion

No aquarium owner wants a setup with brown, decaying plant life. Unfortunately, it’s common for even experienced aquarium owners to see color changes in unhealthy plants. 

Everything from nutrients to lighting conditions can affect healthy growth in an aquarium plant. 

It’s important to provide the proper setup for both plants and fish if you want your entire aquarium ecosystem to thrive. 

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