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Brown Spots on Aquarium Plants — Reasons & Remedies

Brown Spots on Aquarium Plants — Reasons & Remedies

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Aquascaping, or the art of growing a garden of aquatic plants amongst stones, driftwood and a variety of substrates, is becoming an increasingly popular method of plant cultivation. 

When this art is practiced in an aquarium that is also home to fish, it creates beautiful and unique displays.

Combined with well-chosen LED lights that help plants grow, such as colorful lights, spotlights or light blue nightlights that have a “moonlight” effect, aquascapes make beautiful home design features. 

Unfortunately, these underwater gardens become less impressive as displays when the plants in them begin to develop brown spots. 

Brown spots can be caused by poor water quality, water with the wrong pH levels, or a lack of certain nutrients in your tank’s water or substrate. Your plants may also have brown spots because of brown algae growth.


Brown spots on aquarium plants because of problems with water

Aquatic plants have highly specific water requirements. As a result, it is crucially important that you do your research about the pH levels your plants need before planting anything in your tank. 

Plants may become unhealthy and develop brown spots if they are kept in water with the wrong pH levels. 

If you are planting different kinds of plants, you need to ensure that the plants your put in your tank can survive in water with the same pH levels and temperatures. You should also look into whether the plants you choose prefer hard or soft water. 

If your plant’s beginning to develop brown spots or are going brown around the edges, it is worth investing in a pH measure, so you can keep tabs on the pH levels in your water. For more information about pH levels in water, read this handy guide about how to move pH levels up and down in hydroponics

It is also vitally important that you filter or distill your water, because even trace amounts of chlorine in tap water can cause plants’ leaves to develop brown spots or turn completely brown.

This guide to the best water for houseplants can offer some helpful clarification about water quality. 


Brown spots because of a nutritional deficiency 

Brown spots can also develop on the leaves of aquatic plants when the plants are not receiving the nutrition they need. 

The main macronutrients plants need are phosphorus, potassium, nitrogen and magnesium. Without the correct balance of these, your plants will begin to wilt and may develop brown spotting. 

Plants that do not get enough phosphorous, for example, struggle to grow and will remain small. If the deficiency continues over a long period of time, dark brown spots will appear on their leaves. 

Too much phosphate can also cause brown spots on aquatic plants, so it is crucial that the phosphorous levels in your tank remain within the correct boundaries. 

Potassium deficiency causes small black spots to develop on plant leaves. These spots will turn into holes over time. Potassium-deficient plants also turn yellow and fail to grow to their expected size. 

In addition to the four major macronutrients, plants also require a wide spectrum of micronutrients, including iron, copper, manganese, boron, calcium and zinc. If plants are not getting the mineral elements they need, they may become nutrient deficient and will develop necrosis.

Necrosis is a plant rot that causes leaves to become brown and black. If the symptoms are not reversed, your aquatic plants may die. 

Iron deficiency often leads to necrosis. The best way to prevent your aquarium plants from becoming nutrient deficient is to supplement the water in your tank with an appropriate fertilizer blend. 

A good tip is to choose plants for your tank that have similar nutritional needs so that you can make sure you are meeting the mineral needs of all your plants. 


Brown spots on aquarium plants because of the wrong amount of light

Plants make food through a process known as photosynthesis, in which green plants use sunlight to synthesize nutrients from carbon dioxide and water. 

If plants do not receive enough light, they cannot feed themselves. On the other hand, if plants are exposed to too much light, they will also not be happy. 

Leaf browning and spotting can result from either of these problems.

Using LED lights to grow aquarium plants is the best way to ensure that you are giving your plants the right amount of light. Use a timer to ensure regular and consistent light exposure and make sure that the strength of your LED lights is suited to the depth of your tank. 

As a general rule, you need LED lights with 10–20 lumens per liter for low-light plants, 20–40 lumens per liter for medium-light plants and over 40 lumens for plants with high light requirements. 

You will also need to ensure that your lighting system has good spread, and that none of your aquatic plants are blocking the light from reaching other plants. 


Brown spots on aquarium plants because of brown algae

One thing to be aware of when it comes to the lighting in your tank, is that the more light you give your plants, the more light you are also giving green spot and green water algae, which lovs light and which can cause myriad problems for your aquatic plants.

However, if you give your plants insufficient light, they will grow brown algae, which can cause your plants to rot and develop brown, rust-colored spots on their leaves. 

Brown algae is not actually algae and so cannot be treated in the same way as regular algae. Rather, brown algae spots are tiny colonies of microscopic pests called diatoms. This is good, because it means the spots can be wiped off and will not cause too much damage if detected early enough.

The bad news is that brown algae settles on all the surfaces of your aquarium, and causes everything to look dirty. The good news is that this makes it difficult to mistake brown algae for something else. 

If your plants leaves have brown, rust-colored spots and your tank is becoming cloudy and looks dirty, you can be fairly sure you are experiencing a brown algae infestation. 

Brown algae grow where there is an excess of silicate and nitrates. Silicate can get into your tank if you used undistilled or unfiltered tap water. 

Nitrates are often found in tap water and are also introduced by fish waste and rotting plant matter. 

Fortunately, brown algae is relatively easy to get rid of. Clean your tank thoroughly by wiping the glass from top to bottom. Use a non-abrasive pad and work in a swift sweeping motion to avoid spreading the diatoms around.