Aquatic Plant Nutrition

You’ve got the turtle pellets, the fish flakes, and the shrimp granules, but do you have the liquid plant fertilizer? Not many people keep live plants in an aquarium, and fewer still realize they need to do more than simply plop them in the water. Depending on your aquarium setup, water hardness, and plant species, that may be all you need to do. For most of us, however, you will need to think about your plants’ nutritional needs sooner or later.

Sources of Plant Fertilizer

Aquarium plants can get the nutrition they need from a variety of sources. These are the three big ones:

Fish Food (and waste)

Other than water (and love), this is the stuff you will be putting the most of in your aquarium. Commercially available fish foods are packed with vitamins and minerals to help keep your fish happy and healthy. It should be no surprise that fish and plants require many of the exact same supplements.

Of course what fish eat eventually comes back out again. Think of fish waste as plant fertilizer. We put animal manure on our fields before planting crops right? It works much in the same way. Of course, like anything else, too much of a good thing can be bad. So always monitor your parameters for a happy healthy tank.

If you have a heavily planted aquarium, try to imagine you are feeding “the tank” and not just your fish. Dr. Diana Walstad has written a great book that describes in detail the relationship between fish food and plants. It is called, “Ecology of the Planted Aquarium“. If you haven’t read it, I highly recommend it.

Chemical Fertilizer

There are many brands and types of chemical fertilizers. They come in a variety of combinations, either singular supplements like chelated Iron, or mixed nutrient recipes like Aquarium Co-Op’s Easy Green. All these products simply add vital nutrients to your aquarium. You should carefully read the instructions on the packaging to make sure you do not overdose your tank. Again, too much of a good thing can be bad, very bad.

Tap Water

For those of us who live in areas with hard water, chances are many minerals and salts like Calcium and Magnesium are already abundant in the water. Some places may have water high in other nutrients like Nitrate. Well water is typically very high in trace elements like Iron and Copper. You will have to test your own water or visit the website of your local water authority to know what and how much is available straight from the tap.

Nutrient Deficiency in Plants

Signs of nutritional deficiency in aquatic plants is the same as in terrestrial plants. The color, shape, and texture of the leaves can tell you much about what your plant is lacking. Below is a list of the Macronutrients and Trace Elements your plants need to be healthy as well as how to look for deficiencies.


Macronutrients are nutrients that plants need “the most of”. Without these your plant cannot grow properly (or at all in some cases). Here they are in alphabetical order:

Calcium (Ca)

Calcium is a metal that plants use to build healthy cell walls and transfer materials across those walls. If you have hard water, chances are there is plenty of Calcium for your plants. If you are dosing Carbon, and encouraging faster than normal growth, you may need to supplement Calcium.

Some types of substrate are abundant in Calcium: crushed coral, some gravel, and other specialized substrates. You will most likely not need to supplement Calcium unless your water is extremely soft or you are using R.O. water.

Optimum Levels: 10 to 15mg/L

Symptoms of Deficiency: Lack of Calcium is noticeable in areas of new growth. It can appear as “burned” or dead matter at the edges of leaves. The effect is called “tipburn”.

Carbon (C)

Carbon is the “big one,” and is by volume the most important element in all life (as we know it). Plants use it to build the basic structure of their bodies (leaves, stems, roots, flowers, seeds, etc…). Plants meet their carbon quota through consuming CO2. By photosynthesis, plants break the CO2 molecule into oxygen gas (O2) and Carbon. They then use the carbon to create food (sugar) and grow new tissue. Terrestrial, marginal, and floating plants obtain all of the CO2 they need directly from the air. Submerged plants, on the other hand, must gather CO2 from the water column. Some plants in hard water environments have evolved to break down carbonate compounds to get the necessary carbon. Depending on the plant species, your plant may prefer to get CO2 through sediment by way of roots or from the water column through leaves.

In the aquarium environment, availability of carbon is the big limiting factor of plant growth. Just like how lack of light prevents photosynthesis from happening, the same is true for carbon. Without carbon, the plant stops growing, even if the plant is getting plenty of light. This is why many people who are serious plant-growers dose their tanks with pressurized CO2. This gives the plant abundant carbon to grow. The risk of dosing CO2 in a tank with fish is the same as any other fertilizer: a miscalculation can result in mass fish death.

Optimum Levels: Variable, depends on amount of available light

Symptoms of Deficiency: Lack of carbon results in slow or no new growth. Your plants may begin melting, or appear weak and easily broken. Holes in the leaves may also develop over time. Plants use carbon as their primary building material, without it they simply cannot grow new tissue. A lack of carbon may also mask other deficiencies, because without new growth, the plant can’t produce damaged tissue any more either.

Hydrogen (H)

It’s the water! This is the most abundant nutrient available to your plant. If your plant is suffering from lack of Hydrogen, you may have accidentally planted it in your shoe. But seriously, aquatic plants use H2O (water) to inflate their structures and to transport sugars and other materials around their bodies. Hydrogen is also an important element in the process of photosynthesis.

Optimum Levels: N/A

Symptoms of Deficiency: When dealing with aquatic plants, this is never going to be a problem. If your plant is deficient in Hydrogen, it will be dry and brittle like plants that have been left out of the water too long.

Magnesium (Mg)

Magnesium is important for different functions in a plant cell, but perhaps the most important use is its role as an ingredient in chlorophyll. Chlorophyll is the green pigment that is central to photosynthesis in plants. There are other pigments too, but chlorophyll is the most commonly used, which is why most plants are green.

Magnesium is one of the typical components of hard water. In hard water areas this element should be adequately abundant from the tap. In fact it may be in excess, which could inhibit the plant’s ability to take in other elements from the water.

Optimum Levels: 10 to 15mg/L

Symptoms of Deficiency: Lack of magnesium can be presented by older leaves becoming paler and developing chlorosis between the veins. Chlorosis is when the plant cannot properly produce the green pigment chlorophyll. The leaves will take on a yellow shade while the veins remain green. Leaves may begin to melt or fall off. Leaves that get more light exposure will have more intense symptoms.

Nitrogen (N)

Nitrogen is a useful element for plants to make things like proteins and for use in photosynthesis. The most common source of Nitrogen in the aquarium is from the waste of fish and other animals. If you read my article on the Nitrogen Cycle, you can learn how Nitrogen flows in the aquarium. If you have fish in your aquarium, there should be no lack of Nitrogen.

Optimum Levels: 5 to 30 mg/L

Symptoms of Deficiency: Lack of Nitrogen is represented by Chlorosis of leaves, especially older ones. This causes the leaves to take on a yellow/brown appearance. In an extreme case, old leaves can take on a reddish/brown color. Curling of the leaves may also be apparent. New leaves may not show symptoms at first, because existing Nitrogen can be passed to them during initial growth.

Oxygen (O)

Oxygen is used by plants in a number of ways including respiration and photosynthesis. It is also a waste product of photosynthesis. Plants do not require a high amount of oxygen, and in fact may grow better in lower oxygen environments. This is because Oxygen has a tendency to bond with other elements in the water that the plants need, making them unavailable to the plants.

You may not need to supplement your plant’s oxygen requirement through aeration of the water, but your fish and other pets likely will. If you have few fish in your aquarium, and keep a bubbler primarily for water movement, you might consider only turning it on when the lights are off. This way your fish get the benefit of not being choked by excess carbon dioxide build-up at night, and your plants benefit from not having too much oxygen in the water during the day.

Optimum Levels: N/A

Symptoms of Deficiency: You aren’t likely to encounter a lack of Oxygen without also experiencing lack of Carbon or other nutrients critical to photosynthesis. Plants get all the Oxygen they need by creating it themselves as a product of photosynthesis. A lack of Oxygen can be represented by slow or no new growth.

Phosphorus (P)

Phosphorus is a major component of the energy transporting compound ATP. Most plants primarily obtain phosphorus from the substrate through their roots. Many fish foods contain high levels of phosphates, so as long as you are feeding your fish there should be no lack of phosphorus.

Optimum Levels: 5 to 30mg/L

Symptoms of Deficiency: Lack of Phosphorus can begin with new growth being a very dark green color that grows into a light, bland green. Older leaves can appear to take on a purple coloration. Growth can be stunted and have small dead spots (black or dark brown). Other symptoms can appear similar to those of Nitrogen deficiency.

Potassium (K)

A very important element for many cellular processes, Potassium is primarily absorbed from the water through the leaves. It is a critical nutrient involved in the regulation of CO2 uptake and is used in the production of important molecules for use in photosynthesis.

Potassium is not something that is commonly found in tap water at appropriate levels for plant growth, and it is often quickly depleted by plants in an enclosed system. If you are using a soil substrate there should be plenty available that will slowly leach into the water. Otherwise you may want to dose with fertilizer or add some potash to your substrate.

Optimum Levels: 5 to 15mg/L

Symptoms of Deficiency: Lack of Potassium begins with patchy chlorosis followed by dying tissue. Small holes may develop in the leaves, and these holes will continue to grow, until the leaf dies. The edges of leaves can appear “burned” or “scorched”. The tips of leaves may curl. New leaves will be smaller than healthy growth. These symptoms usually appear on older leaves first.

Sulphur (S)

Sulphur is used by plants for many functions, including as an ingredient in chlorophyll. There is usually abundant sulphur in tap water and even in rain water due to air pollution. You should only need to supplement this element if you are using R.O. water.

Optimum Levels: N/A

Symptoms of Deficiency: Lack of Sulphur appears similar to Nitrogen deficiency. Chlorosis appears first in younger leaves, because Sulphur cannot be moved from older regions to newer ones.

Micronutrients (Trace Elements)

Micronutrients are important for healthy plant growth, but are not needed in the same quantities as Macronutrients. Many of these can be found in ample quantities in tap water and soil substrates.

Boron (B)

Boron is used by plants for many processes including flower production. Adequate levels of boron are generally found in tap water.

Symptoms of Deficiency: Lack of Boron is first seen in new growth. Tips of stems and new leaves will be blackened with necrosis.

Chlorine (Cl)

Chlorine is used by plants in photosynthesis and other cellular processes. This too is often found in adequate levels in tap water, even after using dechlorinator.

Symptoms of Deficiency: Lack of Chlorine is unlikely, and there isn’t much written about it, that I could find, in the aquarium hobby. After a little Googling, I did discover a description of the symptoms in wheat: “Wilting, restricted and highly branched root system, often with stubby tips. Leaf mottling and leaflet blade tip wilting with chlorosis” source: Spectrum Analytic.

Copper (Cu)

Plants use copper to assist in cellular respiration. It is typically abundant in tap water. This too can be found in excess, particularly in older homes with copper pipes and in well water. Too much Copper is toxic to both fish and plants.

Symptoms of Deficiency: Lack of Copper can result in dark green leaves and necrosis on the edges of leaves that eventually works its way to the base of the leaf. Leaves may fall off prematurely and/or be deformed and twisted.

Iron (Fe)

Iron is a very important element for plants. It is used in respiration and in creating chlorophyll. There should be plenty of iron in a soil based substrate, however high oxygen and hard water aquariums can restrict your plants’ access to iron by it being bound to other compounds.

If you do need to supplement iron it would likely be in the form of chelated iron, which is available at most garden or aquaponics suppliers.

Optimum Levels: 0.05 to 0.1mg/L

Symptoms of Deficiency: Lack of iron can be represented by chlorosis or paleness between the veins, beginning at the tip and moving back to the base. It shows first in new growth, since iron cannot be moved out of old growth easily.

Manganese (Mn)

Manganese is used by plants in photosynthesis and the production of chlorophyll. Adequate levels are often found in tap water.

Optimum Levels: < 0.05mg/L

Symptoms of Deficiency: Lack of Manganese results in chlorosis between the veins on leaves. In these yellowed spots, small areas of tan or brown spots may occur. These symptoms appear first in new growth, which may seem slowed or stunted.

Molybdenum (Mo)

Plants use Molybdenum as part of the process to break apart nitrate into ammonium (which they then use for a source of Nitrogen). This nutrient is usually available in tap water, but can also be supplemented. Soil substrates should contain ample amounts of this nutrient. It is especially important in hard water aquariums where ammonium is quickly converted into toxic ammonia.

Optimum Levels: < 0.05mg/L

Symptoms of Deficiency: Because Molybdenum is an important part in the process of handling Nitrogen, a deficiency can appear similar to that of a deficiency in Nitrogen. Chlorosis between veins and premature dead tissue in older leaves can occur.

Nickel (Ni)

Plants use nickel as a component in the process of creating ammonium from other Nitrogen based compounds. There is usually sufficient amounts of nickel in tap water and soil substrates.

Optimum Levels: < 0.05mg/L

Symptoms of Deficiency: Lack of Nickel can result in chlorosis and necrotic tissue at the leaf tips.

Zinc (Zn)

Zinc is an important element in the production of chlorophyll. There should be plenty of zinc in most tap water systems and soil substrates.

Optimum Levels: < 0.05mg/L

Symptoms of Deficiency: Lack of Zinc can result in chlorosis between the veins and around the edges of older leaves.  Yellowed spots may also be present.


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