What Causes Black Spots on Aquarium Plants?

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As a novice aquarist, caring for the fish and crafting a balanced and healthy environment for them will take away most of your focus and energy.

This means you will have little left to spare for managing your plants, but you need to do it.

Aquarium plants don’t need as much care and maintenance as the fish, but they do require some attention.

Otherwise, plants will begin to die off gradually, often displaying foretelling signs of what’s to come. Black spots on the leaves are pretty good indicators of an unwell plant heading in the right direction.

Today, we will discuss plant black spots, the main causes, and how to treat and prevent them effectively.

Causes of Black Spots on Aquarium Plants

Aquarium plants will display black spots for a variety of reasons and they are not all obvious.

The most compelling ones to remember include:

High Nitrate Levels

This is a somewhat paradoxical one because plants tend to use nitrates as their primary food source.

They will consume nitrate and ammonia in varying quantities, but they are capable of processing a lot more of the former. That being said, there is such a thing as too much nitrate.

The problem with too much nitrate is that it interferes with the plant’s iron synthesis. So, excess nitrate equates to low iron intake, causing the plant to experience iron deficiency.

This is generally the first phase, during which the plant will exhibit discoloration via yellowing leaves. The leaves will become white in more severe cases.

Eventually, black spots will form, indicating the nutrient deficiency and warning of the presence of excess nitrate.

The plant may also exhibit signs of fungal or bacterial infections due to the excessive amounts of water nitrate.

In this sense, you have to consider 3 important notions:

  1. Luxury consumption phase – This notion refers to the plant receiving more nitrate than it needs. But only slightly more. In that case, the plant’s metabolic rates will drop to manage the higher nitrate concentrations. The situation is not alarming since plants have mechanisms in place that help them regulate some excess nitrate in their environment.
  2. Nitrate toxicity – This effect occurs when the nitrate levels outweigh the threshold set by the luxury consumption phase. At this point, the plant can no longer regulate nitrate intake and will exhibit signs of toxicity.
  3. Different needs – Not all plants require the same amount of nitrate. It all comes down to the type of plant and how stocked the aquarium is.

So, as you can see, the nitrate aspect isn’t as easy-to-grasp as it might seem.

High Phosphate Levels

Phosphate is the byproduct of decaying matter, including fish food leftovers, plant matter, dead fish, fish waste, etc.

Unlike nitrates, which are allowed in the water under certain limits, phosphates are dangerous even in low concentrations.

This problem is easily manageable via adopting a healthy tank maintenance routine consisting of cleaning any waste and keeping the tank water in pristine condition.

A water filter is necessary to provide mechanical and chemical filtration while supporting the tank’s biofilm.

The presence of the beneficial bacteria will work against dangerous chemicals like ammonia, nitrites, and phosphates that can hurt the fish and plants.

Excess Light

Plants require light to fuel the process of photosynthesis. The problem is that too much light can actually be detrimental to both fish and plants.

Excessive lighting can cause the plants to overuse the CO2 in their habitat, leading to CO2 scarcity and impacting their ability to process nutrients and perform photosynthesis.

But there’s another problem as well. Algae thrive in well-lit environments. The algae overgrowth that will take over the tank will come with a new set of problems, making the situation even worse.

I recommend learning about each plant’s light requirements and relying on LEDs to fulfill their needs.

Black Beard Algae

Black Beard algae are considered pests in any aquatic environment. They are ugly, will spread fast, and can cover any surface, including the plants.

Black Beard algae aren’t dangerous in the sense that they don’t release any chemicals, nor do they foul the water.

The problem is that Black Beard algae overgrowth will cover the plants and limit their access to sunlight.

They will also consume nutrients from the water, impacting the entire environment eventually.

So, if you notice black spots on your plants, look closely. You may be facing Black Beard algae, in which case you need to act fast.

These organisms will spread fast, disrupting the habitat’s balance. Learning what fuels the Black Beard algae to grow is key to countering them.

The ideal environment for Black Beard algae is one with low CO2 levels, excess of water nitrates, and excessive lighting.

Dirty waters and lack of tank maintenance are additional factors that contribute to algae formation and spread.

High Temperature

Most aquatic plants require water temperatures up to 84 F on average. Some may go higher, others lower.

For the most part, though, tank plants prefer cooler waters, up to 80 if possible. The problem is that it’s easy to forget to monitor the water’s temperature.

This happens mostly in fishless tanks in environments based on plant-oriented aquaculture.

The presence of the fish will keep aquarists on their toes, forcing them to monitor water temperature and other parameters constantly. That awareness gets dialed down a bit when it comes to plant-only tanks.

As a result, the water temperature may increase, hurting and even killing the plant. Plants will react to warmer waters similar to any other organism.

Their metabolic rates will accelerate accordingly, causing the plant to grow faster and larger. If the water is too warm, though, the plant will begin to die off.

The leaves will turn brown and even display black spots, then fall off as the plant dies.

You should always monitor the tank water temperature to prevent that. And invest in a heater to keep the temperature stable.

How to Remove Black Spots from Aquarium Plants?

Fortunately, you have several ways of removing black spots from your aquarium plants, depending on what triggered the problem.

These include:

Manage Phosphates

As we have already discussed, phosphates are the direct result of fish waste and decaying organic matter in the tank water.

If the level of phosphates is too high, you may have too much waste in the tank.

Remove any fish waste, food leftovers, algae deposits, and plant matter that may influence the water’s chemistry.

You should also perform a 20-25% water change and verify the phosphate levels to see where you stand. If the damage isn’t significant, the plants will bounce back shortly.

If they don’t appear to recover, despite neutralizing the phosphates, the problem may be elsewhere.

Or the plant has already reached the point of no return, in which case you might as well remove it from the environment.

Manage CO2

This is a crucial point since CO2 is the building block for aquarium plants and plants in general.

If your tank water showcases low levels of CO2, consider fixing the problem soon. Otherwise, your plants will start dying.

To restore the CO2 levels back to normal, consider the following fixes:

  • CO2 injections – This method is among the most popular for a very simple reason: it fixes the problem immediately. There is no waiting time involved since the CO2 will increase to the desired value on the spot. We’re talking about using a pressurized canister that will deliver the CO2 into the water via a needle-like valve. Hence, the ‘injection’ descriptor.
  • CO2 supplements – A pressurized CO2 system is quite expensive, so not everybody can get it, especially if the tank is rather small. You don’t want a high investment only to boost the CO2 levels in the water. The good news is that you also don’t need one. There are a variety of CO2 supplements available at reasonable prices that will get the job done fast. Find the one that best fits your tank set up and use it according to the instructions on the label.
  • DIY variations – There are some recommendations in this sense, like using yeast and sugar, which will create alcohol and CO2 when mixed with water. This is a professional kit to get if you’re not really familiarized with a CO2 injector and like to keep things simpler. The yeast-based system is great for beginners due to its ease of use and reliability.

When it comes to plants, there really isn’t an upper CO2 limit to be worried about. Plants can’t really overdose on CO2, for the most part.

However, excess CO2 can kill the fish and other aquatic animals you may be housing.

I recommend monitoring the CO2 levels in the water and keeping the gas within the safe parameters.

Going overboard with the CO2 won’t add extra benefits to your plants. They will only use what they need.

Combat Black Beard Algae

This is an important one because Black Beard algae are a true pest of any aquatic environment.

Getting them is easy, but dispatching them can be a real pain. Black Beard algae will grow and spread fast under the right circumstances.

This means you should combat them fast before they get out of hand.

To do that, you must first understand what’s causing Black Beard algae to emerge in the first place.

There are several conditions that will lead to the emergence of Black Beard algae in pretty much any tank:

  • Too much lighting
  • Excess of nutrients, especially nitrates
  • Low CO2 levels creating the ideal condition for the algae to thrive
  • Improper water conditions with a lot of dead matter

Black Beard algae aren’t dangerous directly since they don’t compete for the same nutrients as plants, and they don’t alter the water’s chemistry.

The dangers appear rather indirectly due to the algae’s predilection towards growing on the plants’ leaves. They will also cover other surfaces, but it’s the leaves that they prioritize.

If the conditions are adequate, the algae will cover the plant completely, restricting its access to sunlight.

This will deprive the plant of the energy necessary to perform photosynthesis, which will kill it.

To prevent all these problems, you need to combat Black Beard algae immediately.

You can do that via several means, such as:

  • Relying on algae eaters – A variety of fish will consume Black Beard algae. These include Siamese algae eaters, Flying Fox, American Flagfish, Black molly, the goldfish, cherry, and rosy barbs, etc. You can find a variety of fish that have algae in their diet, but they will consume it at various rates, so they’re not all equally effective. Also, fish can’t consume fully mature algae that have already spread everywhere. They will eat the younger ones, which means that algae eaters are good at preventing the algae’s spread. But they’re not good at eliminating it once it has taken over the environment.
  • Hydrogen Peroxide – This is an effective solution to an algae problem, especially the Black Beard type. The treatment consists of removing the affected plants and decorations from the tank and treating them with the solution. If the algae are too spread out, consider treating the entire tank water. The hydrogen peroxide won’t harm the fish or the plants. Just to be safe, you should move the fish into a separate tank during the treatment.
  • Remove the algae manually – This is a pretty good approach since it allows you to contain the problem with minimal intervention. The procedure is as simple as it sounds. You only need to use a sponge to clean the tank walls and aquatic decorations that show signs of Black Beard algae. Do the same for the plants. Clean the leaves, and the algae will spread into the water, which will allow the filtration system to pick them up.

The Black Beard algae aren’t such a severe problem, despite being quite common in the aquarium business.

The idea is to tackle them as soon as they appear and prevent them from spreading. In this sense, I recommend getting some algae eaters in your tank just to be safe.

These fish are always on the prowl for algae, so they will contribute to a healthier and most stable aquatic environment with minimal intervention on your part.

How to Prevent Black Spots on Aquarium Plants?

I would say that there are 3 fundamental methods that will help you prevent plants from developing black spots:

  1. Ensure a stable and thorough tank maintenance routine – This involves cleaning any residual dead matter than may change the water’s chemistry and performing regular water changes. Doing so will stabilize the environment and keep the water cleaner, fresher, and more oxygenated for longer.
  2. Prevent algae buildup – Clean the tank regularly, make sure that the water is fresh, keep the lighting moderate, and remove any algae deposits whenever necessary. These methods, in combination with having some reliable alga eaters, will keep the algae in check long-term.
  3. Manage CO2 – For the most part, CO2 will be synthesized naturally in any aquatic environment. CO2 is the result of bacterial activity, and plants will also produce it during nighttime. Fish also produce CO2. However, it sometimes may not be enough, especially if your tank is heavily planted. You should always monitor the CO2 levels and supplement the tank whenever necessary. Doing so will keep the plants growing and boost their coloring.

In addition to all these measures, you should also monitor water nutrients, manage the temperature, and keep the nitrate levels relatively low.


Plants will develop black spots occasionally, but there’s nothing to fear. In most cases, the solution is rather simple and will reverse the process soon.

If your plant doesn’t recover, consider removing it from the environment.

Otherwise, it will decay in the water, boosting ammonia and fouling the environment.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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