You may have heard that live plants are great to add to your freshwater aquarium to help with algae problems. But why?
Plants take up nutrients, like nitrate, from the water to grow. Not only does this nitrate reduction method help reduce the number of excess nutrients in the aquarium water column, but it eventually introduces more oxygen for fish and invertebrates, too.
Keep reading to learn everything you need to know about how plants use excess nitrate content in the aquarium and which species are the best at keeping your water clean!
Plants and the nitrogen cycle
All aquarium plants with green leaves need nutrients. The three primary nutrients include nitrogen (N), phosphorus (P), and potassium (K). They also need secondary nutrients, like calcium (Ca), magnesium (Mg), and sulfur (S).
Each of these nutrients serves the plant to promote growth, overall health, and reproduction. But how do plants get these nutrients?
Plants use the photosynthesis process to make food, turning carbon dioxide, water, and light into oxygen and sugar (glucose). They use their roots and other specialized areas to absorb and transfer the additional nutrients needed for these processes.
Most types of terrestrial or potted plants get these nutrients from the soil through their roots, which is why farmers fertilize their land; the same idea carries over to the world of aquatics!
As mentioned before, plants use nitrogen, namely in the form of ammonia (NH3), ammonium (NH4), nitrite (NO2), and nitrate (NO3). If this is starting to sound like the nitrogen cycle, then you’re right!
Aquarium plants actually take up and use the ‘bad’ nutrients that your fish and invertebrates can’t tolerate. Because of this, they are sometimes used to help cycle a fish tank and create a natural environment for your fish.
Do plants cycle a tank?
Plants won’t initiate a cycle in aquariums, but they will usually help speed it along as the plants use those harmful nutrients for growth.
Theoretically, if enough submerged plants are added to the aquarium during a cycle, the nutrient uptake will be faster and greater than the ability to test for ammonia, nitrite, and nitrate reduction.
This is known as a silent cycle and is a natural biological process. The conversion of ammonia to nitrite to nitrate by beneficial bacteria can’t be seen through testing even though the biological cycle is happening. Instead, the plants are using them before they can be detected.
It can be pretty confusing to watch and difficult to know exactly when the natural nitrogen cycle is completed.
In this way, expert hobbyists can quickly set up densely planted tanks and introduce fish as long as the bioload is less than the rate at which aquatic plants use nutrients from fish waste (plus any additional water changes for extra security).
Likewise, hobbyists can set up an aquarium with plants using the Walstad method, a self-contained biological ecosystem without the need for external filtration.
How do plants remove nitrates?
In simple terms, more nitrates allow individual plants to grow more through photosynthesis.
Nitrate is used to produce amino acids, the backbone of proteins. These proteins may be used for metabolic processes or storing sugars and other foods throughout the plant.
Nitrates are also needed for the healthy production of chlorophyll, which are pigments that capture energy from the sun. Without these nitrates, chlorophyll production decreases and can cause yellow and brown leaves.
As mentioned before, plants also use ammonia, ammonium, and nitrite for other physiological processes.
While it might seem like loading your system with nitrates will cause healthy exponential growth, it can also cause algae outbreaks. As we’ll discuss, algae thrive in waters with high and low nutrient levels.
Nutrient input should be balanced with nutrient output to keep growth steady. Finding the right balance of plants and fish is the key to a happy aquarium. Water changes should also be performed as needed.
What is the ideal nitrate level for a planted freshwater aquarium?
The ideal aquarium nitrate level for a planted freshwater fish tank is below 40 ppm. Any higher than this in the aquatic environment can lead to algae problems and cause trouble for your fish and invertebrates.
There is a tricky balance between available nutrients, livestock, and plant growth.
Plants need nutrients to grow, but not so much that algae are attacking them and stunting your plants. Algae thrive on three things: nitrates, phosphates, and light.
As most algae species grow faster than other plant life, excess nutrients will likely lead to unwanted algae growth. On the other hand, low nutrient levels can also cause algae growth and poor plant growth.
You need to regularly feed and stock enough fish to have a constant input of nutrients into your system. Water changes can quickly remove large amounts of nitrate from the water column, and it’s essential to understand how they’re being replaced.
You never want your nitrate concentration to drop below 10 ppm in a planted aquarium. This stunts plant growth, invites advantageous species of algae, and induces asphyxiation for plants.
In short, you always want your nitrate levels to be between 10-40 ppm. If your nutrient input is less than what is being used, it will be necessary to dose fertilizers.
The best nitrate-removing freshwater plants
Though all plants use nitrate to grow, some plants use it faster than others.
If you have a particularly large bioload in your aquarium or just want to minimize tap water changes, then there are a few species of live plants that might be to your benefit.
Here is a list of the best nitrate-removing live plants suitable for your freshwater aquarium! None of these plants require high lighting or CO2 injections and they will all help produce a healthier environment in your aquarium.
Duckweed (Lemnoideae subfamily)
Duckweed is a type of floating single-leaf plant similar to a lily pad commonly found on the top of slow-moving lakes, ponds, and even smaller rivers with low water flow.
Duckweed is prolific, and most of the aquatic community think it is a terrible weed. On the other hand, some aquarium systems refuse to sustain any duckweed, which could be a blessing.
This freshwater plant does well under most lighting, though higher intensities can burn the plant due to close proximity. Within a couple of weeks, duckweed can reproduce and cover the entire surface of the aquarium water, providing a low-light environment for bottom-feeding fish.
While this is great for nutrient removal, it can also limit gas exchange and light penetration to the rest of the aquarium. However, it serves as a great spawning area for many species of fish.
Interestingly, this plant has many biotechnical benefits and is being harvested on a large scale and closely studied as a potential source of food for humans.
Frogbit (Limnobium laevigatum)
Frogbit is very similar to duckweed in appearance and nitrate uptake.
These floating plants are circular in shape but have thicker leaves. They are root-feeding plants and their extremely long roots help take up all those nutrients! They are an excellent choice for your larger tank.
Frogbit is a highly forgiving, fast-growing plant and an excellent nitrate absorber that is particularly favored among Amazonian biotopes. Fish of all shapes and sizes can find shelter in their long roots, though some extra maintenance will be needed to keep those roots out of filter and wavemaker intakes.
Otherwise, frogbit is a good beginner-friendly water plant that needs little to no extra maintenance as long as growth is controlled.
Water lettuce (Pistia spp.)
If you’re looking for a more ornate floating plant species, water lettuce definitely makes a statement. Its long leaves make it one of the biggest nitrates-treating aquarium plants available.
This popular aquarium plant has a very delicate appearance, often resembling a head of lettuce. As a hardy plant with attractive leaves and a fast growth rate, water lettuce is especially popular among pond enthusiasts.
Dwarf water lettuce, Pistia stratioes, is particularly favored for its small, containable size.
Hornwort (Ceratophyllum demersum)
Hornwort, also known as coontail, is a very fast-growing bushy plant commonly found in freshwater lakes and ponds. This attractive plant can be placed in the substrate but has no problem establishing itself as a floating plant. Its numerous stems give it the best chances of growth in a healthy environment.
Hornwort will quickly take up excess nutrients but can easily block your light source from the rest of the aquarium. On top of that, it can grow in dense mats that can be difficult to remove if left uncontrolled.
On the other hand, these mats can be used by many species of fish fry and even tadpoles for protection; older adults like goldfish may even like how hornwort tastes as a natural source of food.
The fast growth rate of hornwort makes it one of the few compatible plant species for goldfish, as it can grow faster than it can be eaten.
Water wisteria (Hygrophila difformis)
Many hobbyists hate water wisteria as it can quickly take over a tank, but we love it for its full, lacey appearance and its ability to improve water quality.
Interestingly, the appearance of the mature plants’ leaf shape will depend on the light level they are kept under and the current water parameters, which affect their growth pattern. Brightly lit aquariums will produce long, pinnate leaves, while low light will cause the plant to grow larger leaves. It is often confused with Water Sprite.
Water wisteria is another fast-growing plant that does well as a midground and background plant in lush aquascapes. It has a habit of leaf melt when first introduced to a new environment but quickly acclimates. These aquatic plants are incredibly hardy and fast-growing but without frequent trimming, just a couple of plants can start to choke out other plants due to their rapid growth rate.
Like hornwort, many plant-loving fish will appreciate having this species to munch on in the aquarium.
Anacharis (Elodea spp.)
Admittedly, anacharis isn’t the most beautiful plant on this list, but it’s a very efficient natural filter for unwanted nutrients. On top of that, it is very easy to find these beginner-friendly plants and often they are one of the cheapest plants available.
This South American plant can be grown planted in the substrate or left floating. Goldfish will find this plant especially appetizing and an excellent source of additional food!
It’s the perfect plant choice for beginners with an aquarium on a budget.
Pothos plants (Epipremnum aureum)
Pothos plants don’t exactly fit in this list of aquatic plants, but many hobbyists have successfully used it as a quick way to remove excess nitrates from the aquarium.
This hardy plant can bring an exotic twist to your aquarium as it cascades down the side of your fish tank. Simply take a small piece of pothos, secure it to the side of the tank with the roots submerged, and watch as it quickly grows out and over your freshwater aquarium!
It is important to note that this house plant will take up more space than submerged types of plants as you need to allow more height and width to your aquarium setup, but it is very efficient at removing excess nutrients.
The only extra maintenance needed is regular trimming and pruning.
Problems with fast-growing plants
Though fast-growing plants can be a significant help for nutrient problems, they can also cause a few issues of their own.
Think about it. For plants to use those nutrients and grow, there needs to be a constant input of nutrients replenishing them.
This can be done in two main ways, though both methods are usually combined. One way is adding more fish, and another way is through fertilizing.
Adding fish for nutrients
One solution to a low nutrient tank, also referred to as a clean system, is adding more fish and invertebrates so long as your tank size allows.
This is the most natural way to feed aquarium plants but can be difficult to balance at first. Uneaten food is an excellent nitrate source and can overload your system if you let it accumulate.
Fish make a lot of waste, but this still might not be enough to meet the high demands of a heavily planted tank. In the meantime, you will probably run out of room to comfortably house fish or overload your tank with nutrients too quickly.
Adding fertilizers for nutrients
The better, though more advanced option, is to supplement with fertilizers. This can be intimidating at first, and it will take some time to familiarize yourself with your tank’s water chemistry.
You will need to understand how nutrients move throughout the system, from fish to plant. Using this information, you will see which nutrients to dose, how much, and how often.
For more information about determining if you need to use the estimated index (EI) method or the perpetual preservation system (PPS-Pro/Classic) method for dosing your aquarium with fertilizers, check out our full article here.
The three main fertilizer options are liquid fertilizers, dry powder fertilizers, or root tabs. There are benefits and disadvantages to each, though dry powder fertilizers give the best control over water parameters and are the least expensive in the long run.
Conclusion
For a long time, nitrates were seen as a bad thing to have in the aquarium. The addition of a batch of plants may help keep your aquarium environment healthy, provide an additional natural food source for some species, and keep your fish happy in a higher-oxygen environment.
In fact, ammonia, nitrite, and nitrate are important to always have in the aquarium in one form or another. Nitrates are essential for moderate growth rates and overall health.
If you have any questions about how plants use nutrients, dosing fertilizers, or have had to make a clean system dirty, don’t hesitate to leave a comment below!
