Does adding more iron make my red plants redder?

One of the most common myths in the planted aquarium hobby is that adding more iron fertiliser will make red aquarium plants redder. However, this is also one of the most widely spread myths in the hobby - as with most myths and legends, it starts with a modicum of truth.

When the aquarium hobby was less sophisticated in the pre-internet era, many planted tanks suffered from a general lack of access to nutrients. Liquid nutrient fertilisers and aquasoils were not as widely used. In a planted tank with no nutrient supplementation and no soil base, plants could obtain certain nutrients such as nitrogen and phosphates from animal waste - and potassium/magnesium from some tap water, but such tanks were often chronically deficient in iron - as neither tap water nor fish waste provided it in significant quantities. It was at this time that aquarists realised that dosing liquid fertilizers containing iron improved overall plant health, and the improvement in red plant colour was particularly noticeable. And so the idea was born that adding iron to a tank would improve the colour of red plants.

Nowadays, iron is widely available in aquasoil substrates and most liquid fertilisers contain good levels of it. So for people who dose their tanks regularly, there is rarely a shortage of iron. It is by far the most common element in aquarium supplements on the market.

For the vast majority of people who already dose a liquid nutrient supplement into their planted tank, adding more iron than required will not stimulate the red plant to produce more red pigment. Iron merely enables the production of pigments; it is not the key factor that stimulates pigment production.

If you are not adding nutrients regularly and do not have a soil base in your aquarium, then adding an iron supplement may well improve plant colouration as well as general plant health. However, there are many other factors besides iron that affect plant colouration, so you will need to provide for all these other needs as well.

The large majority of aquarium plant pictures on this site were grown in a tank dosed at 0.03ppm Iron per day (using APT complete). 

Role of iron in plants

Iron is responsible for a wide range of metabolic processes, including the formation of chlorophyll and other pigments. In nature, high levels of iron are found in the soil. Iron is one of the most abundant elements in the earth's crust. However, iron is a micronutrient and plants use a very small amount of it compared to macro-nutrients such as nitrogen or potassium. An aquatic plant uses about 100 times more nitrogen than iron for growth.

What factors can make my red plants redder?

Light intensity (PAR) and spectrum play a major role in plant pigmentation and visual appeal. Using higher light intensities is similar to giving a plant a sun tan; coloured plants often produce protective coloured pigments when exposed to strong light. Using strong red/blue light can stimulate pigmentation in coloured plants and also give red plants a stronger visual colour tone.

Many of the aquarium plant images on this site have been grown at much higher PAR levels than the average planted aquarium. Correcting for the underwater factor of 1.3, the stem plants in this arrangement are receiving 270-300+ umols of PAR.

Overall plant health is extremely important - unhealthy aquatic plants are plants that will not develop good colouration. So it is necessary to provide the full range of nutrients required by plants, not just iron - this includes the various trace elements and other macro-nutrients such as phosphorus/magnesium etc. Carbon dioxide, mainly provided by carbon dioxide injection, also plays a vital role. You will never find a low tech tank that is as richly coloured as a high tech CO2 injected tank due to the limitations of carbon dioxide levels.

Rotala 'Blood Red' in a CO2 injected tank. This plant will grow well without carbon dioxide injection, but the colour will not be as deep red.

To read more in detail how else to boost red plant coloration, this article goes into deeper detail.

What are signs that there may be actually an iron deficiency in my aquarium plants?

Iron is an immobile nutrient in a plant. This means that plants cannot easily move it once it is fixed, so iron deficiency will always show up in newer growth. With mobile nutrients such as nitrogen or potassium, plants can move them from older leaves to newer leaves if the supply is insufficient - this will result in deterioration of the older growth rather than the newer growth.

If you are using an inert substrate, have not added liquid fertiliser (or have only added components such as NPK rather than a complete mix) AND are noticing colour loss in the youngest leaves, your plants may be suffering from iron deficiency. Keep in mind that there are many other non-nutrient factors that can cause a plant to lose colour as it grows - including the stress of adapting to a new environment, a change/reduction in lighting, or even being shaded by a more aggressively growing plant to the side.

Should I add more Iron if I’m using APT complete?

All APT fertilisers contain more than enough iron to support even the most densely planted aquascapes. If you want to increase colouration further, look instead at improving the light spectrum, increasing CO2 and improving flow.

For some species (Rotala rotundifolia and its variants colorata/H'ra/etc, Rotala Goias, Hygrophila pinnatifida, H. araguaia, Ludwigia arcuata, L. brevipes, etc.) remember that they need low nitrate levels in the water column to become redder. This is a classic example of how adding more food (in this case nitrogen/nitrates) will make the plants greener.

To read more about whether nitrate limitation makes red plants redder, go to here.