Understanding plants and their needs

In order to better understand the needs of your aquarium plants, we'd like to take you on a short plant journey. Like all living creatures, aquatic plants need certain conditions in order to live and grow. There are three key factors:

  • The physical environment, including light and temperature
  • The biological environment, including fungi, herbivorous fish or digging fish
  • The chemical environment, including macro- and micronutrients

When it comes to fertilising plants, let's take a look at the macro- and micronutrients. Plants make use of biologically available compounds as appropriate in order to absorb a wide variety of elements. Carbon dioxide (CO2) is used as a source of carbon (C), for example. In the table below, we explain the different elements that should be added when fertilising.





  • Biologically available forms



Deficiency symptoms




Carbon, C

  • CO2
  • HCO3
  • Basic element of all life
  • No carbon source means no photosynthesis → Depleting energy reserves → Plant death

Ensure sufficient carbonate hardness (> 5 °dKH) in combination with pH values between 6.5 and 8.5; fertilisation with CO2 possible


Nitrogen, N

  • NH4+
  • NO2
  • NO3
  • Component of chlorophyll
  • The most important nutrient for the formation of amino acids and protein
  • Metabolism disorder → Protein depletion → Poor root growth → Discolouration and death of leaves

Fertilisation depends on stocking density and feeding behaviour:
minor supplement recommended


Phosphorus, P

  • PO43−
  • HPO42−
  • H2PO4
  • Cell building block
  • Important for control of cell functions
  • Energy source
  • Chlorophyll depletion, increased anthocyanin content → 
    Leaf discolouration (autumn foliage)

Fertilisation depends on stocking density and feeding behaviour:
minor supplement recommended only with dense planting


Potassium, K

  • K+
  • Controls water supply
  • Disrupted water supply → "Wilting"
  • Reduced assimilate transport → Yellowing from the outside to the inside of older leaves

Supplement required


Sodium, Na

  • Na+
  • Regulates the efficient use of water
  • Na ions activate plant metabolism
  • None known

Sufficiently present in tap water


Magnesium, Mg

  • Mg2+
  • Central building block of chlorophyll
  • Contributes to the formation of proteins, carbohydrates and vitamins (part of metabolic processes)
  • Disrupted photosynthesis → Lighter patches on leaf

Supplement required 


Calcium, Ca

  • Ca2+
  • Structural and functional element for cell membrane
  • Disrupted growth of younger leaves → Young leaves faded and yellow

Supplement required


Iron, Fe

  • Fe2+
  • Activates enzymes
  • Function in seed and germ formation
  • Participation in photosynthesis
  • Reduced chlorophyll synthesis → Yellowish leaves with green veins

Supplement required


Sulphur, S

  • SO42−
  • HSO4
  • HS
  • Building block of amino acids that are essential for chlorophyll supply
  • Important for the use of nitrogen in metabolism
  • Decreased formation of chloroplasts and chlorophyll → Lightening of youngest leaves
  • Decreased metabolic activity, disrupted protein synthesis → Stunted growth

Minor supplement recommended


Chlorine, Cl

  • Cl
  • Controls water supply

None known


Sufficiently available, should not be added unnecessarily




Micronutrients (trace elements)



  • Biologically available forms



Deficiency symptoms




Manganese, Mn

  • Mn2+
  • Activates enzymes
  • Promotes carbohydrate and protein synthesis
  • Important for hormonal balance
  • Too much or too little present → Leaf lightening, colour changes in young leaves; spots later die off and leave small elongated holes

Supplement required


Molybdenum, Mo

  • MoO42−
  • Activates enzyme metabolism
  • Constituent of nitrogenase
  • Important for energy metabolism
  • Decreased growth → Chlorosis formation, lightening of older leaves, leaf deformations

Supplement required


Boron, B

  • HBO32−
  • H2BO3
  • Component of the pectins in the cell wall
  • Affects functions in carbohydrate metabolism
  • Important for cell division, cell differentiation and cell stretching, cell wall stabilisation, tissue formation
  • Prevents cell building and cell development → Reduced root growth, changes to young leaves

Supplement required


Copper, Cu

  • Cu2+
  • Component of protein synthesis and photosynthesis
  • Responsible for stem stabilisation
  • Activates enzymes
  • Prevents cell division and inhibits photosynthesis → Youngest leaves roll up and die


Supplement required 


Zinc, Zn

  • Zn2+
  • Increases disease resistance
  • Component of enzymes and influences enzyme reactions
  • Various secondary diseases such as inhibited growth, leaf discolouration or leaf deformation

Supplement required


Cobalt, Co

  • Co2+
  • Nitrogen binding 
  • Enzyme activator
  • Central element in vitamin B12
  • Decreased nitrogen uptake → Leaf colour change, reduced growth (symptoms as with nitrogen deficiency)

Minor supplement 


Nickel, Ni

  • Ni2+
  • Central element in nitrogen conversion
  • Reduced nitrogen utilisation  → Darkening and death of the leaf tips


Minor supplement


Iodine, I

  • I
  • Stimulates growth
  • A component of defence mechanisms
  • No known signs of poisoning at very high concentrations

Minor supplement


Many more elements could be added to the list. Titanium (Ti) and chromium (Cr), for example, are not essential, but their effect is astonishing. They significantly affect the colour intensity of both emersed (air) and submerged (water) plants.

In terms of fertilising technique, you should be familiar with the law of minimum and the law of optimum in order to provide your plants with the best possible care and prevent algae growth. Don't forget: any nutrients that are not absorbed by the plants are available to potential algae.


The law of minimum

This law states that it is the scarcest nutrient which determines overall nutrient intake and conversion by any living being. For example, if iron is proportionally the least available nutrient, the absorption of all other nutrients is reduced/limited accordingly. 

The following graphics will help illustrate the law of minimum. The first graphic shows the initial situation. You can see various elements (x-axis), the light factor (orange) as well as the corresponding requirements (y-axis) of the plant. The optimum demand (blue) and the quantity available to the plant (yellow) are shown. In our example, the difference between optimum and available quantity is proportionally highest for CO2.


Illustration 1:

Figure two shows the effects of the initial situation. The absorption of all essential elements is restricted in relation to the least available element (CO2). The actual intake decreases proportionally (purple).

The range between purple and yellow is available to potential algae, as this cannot be absorbed by the plant.


Illustration 2:

The law of optimum

The third figure illustrates the optimum law and shows how it can be exploited in the domestic aquarium. A plant can only convert a limited amount of nutrients (the optimum, shown in blue in the example) in order to achieve its highest growth rate. Everything else remains in the water.

This condition is not practically feasible in community aquariums, so the goal should be a uniform "throttling" of all nutrients. When this is achieved, there are no deficiency symptoms and algae growth is greatly reduced. All elements are proportionally throttled (the ratios between optimum and available demand are the same).


Illustration 3:

Composition of an aquatic plant

The "Redfield" ratio, named after the man who discovered it, provides a guide to the needs of aquatic plants, in that it describes the atomic composition of phytoplankton (C, H, O, N and P). Over time, it has been supplemented by further elements: 

(C106H263O110N16P1)1000Fe8Mn14Zn0.8Cu0.4Co0.2Cd0.2 [xy]

This composition allows us to infer several different facts:

  • CO2 and H2O are the most important "foods" for plants. These are the most common.  →  (C106H263O110N16P1)1000
  • The ratio of N to P is 16 : 1. However, since the water contains both nitrate (NO3) and phosphate (PO4) compounds, a ratio of 11 : 1 must be maintained.
  • Although the trace elements are required in very low concentrations, they are still an important component.  →  Fe8Mn14Zn0.8Cu0.4Co0.2Cd0.2[xy]

The elements above are found in the natural habitat of aquatic plants. It is therefore vital to add trace elements such as copper or zinc in an artificial system such as an aquarium.