Algae 101: what actually triggers algae in a planted tank

Algae is so prevalent in the hobby that many aquarists have given up trying to eradicate it and simply accepted it as part of normal aquarium life. We assure you that it is perfectly possible to have planted aquariums free of visible algae - especially filamentous green algae.

It takes a combination of factors to get a tank to this stage; algae will take advantage of any weakness in a set-up - in this sense it also acts as a signal of what is being done wrong or right.

Algae is opportunistic

The interactions between algae in a planted aquarium are complex and best explained by analogy. Much like weeds in the terrestrial world, algae are opportunistic and will bloom when the opportunity arises. Higher order plants out-compete weeds for space and light. In a dense forest, undergrowth gives way to taller trees that block out light by forming a dense canopy. Treefall gaps in crowded jungles give a chance for the undergrowth to spawn; for young trees to race to fill the canopy gap.

In aquatic ecosystems, our aquarium plants are the equivalent of large trees in the jungle, occupying most of the living space - and many aquatic plants can also grow emersed, gaining access to oxygen at the surface and shading the water below. Algae are at a disadvantage in this situation and have evolved to bloom opportunistically only when they sense a gap in the system.

What actually triggers algae?

While appearing calm and motionless on the outside, complex biochemistry takes place inside the plant as it produces carbohydrates, sugars and other metabolites through photosynthesis. Plants are leaky organisms, and aquatic plants excrete some of their excess metabolites directly through their leaf surfaces and edges. Algae feed on this organic waste. When plants are under stress, for example when adapting to a new environment, which requires a significant amount of reprogramming of proteins and enzymes, the excretion of waste proteins and metabolites is increased.

For example, if you move a plant from a high CO2 environment to a low CO2 environment, the plant will need to divert more energy to CO2 uptake functions to compensate for the lower CO2 levels in the environment. The plant will activate more proteins associated with CO2 capture, while discarding enzymes and proteins responsible for growth functions. Older waste protein/DNA material is shed directly to the leaf surface as the plant reconfigures its internal machinery to make best use of what is available in the current environment.

This is why plants newly introduced into a tank from a source with different growth conditions are often affected by algae, especially on the older leaves. Algae feed directly on these organic waste materials. This organic waste includes ammonia and other nitrogenous waste.

Ammonia and nitrogenous waste are common algae triggers. Other sources in a tank include heavy use of ammonia/urea based fertiliser in the water column, waste from livestock or decaying vegetation. Therefore, good filtration and a biologically mature tank, where organic waste and ammonia are rapidly broken down, will help to reduce trigger factors. Light is only an accelerant - reducing light will reduce the rate of algae growth, but it is rarely the cause of algae growth.

Conversely, when plants are healthy, they produce defensive antimicrobial chemicals and enzymes to ward off pathogens and algae. Healthy plants with enough excess energy to produce defensive chemicals are very resistant to algae. When a plant is deprived of nutrients or does not have sufficient access to CO2, it tends to be weaker and less robust, with less energy to produce these defensive chemicals, making it more vulnerable to algae.

In a planted tank, unhealthy plants are prime algae triggers. So the first step to any algae issue is to pay attention to plant growth parameters - Nutrients, CO2, flow and make sure that each plant is getting what it needs to have healthy growth.

From an evolutionary point of view, attaching to a damaged plant that is not growing well maximises the survival potential of the algae. A healthy growing plant will shade and crowd out the surrounding areas, whereas a dying plant will not. Algae are finely tuned to bloom when plants are weak.

In a aquarium filled densely with healthy, robust plants, algae will find it hard to find a foothold. 

Managing algae by creating an algae resistant tank environment

An aquarium with a large percentage of its surface area planted is a more algae-resistant environment than a sparsely planted tank. Similarly, a densely planted aquarium is more resistant to weeds because the weeds find it difficult to take root. An aquarium full of healthy, growing plants, with each plant defending its leaves with anti-pathogen chemicals and threatening to shade surrounding areas with new growth, is a very hostile environment for algae.

The more sensitive/choosy a plant is, the more likely it is to suffer stress and trigger algae. A tank with simpler, larger, more robust plants will be much easier to keep free of algae than a tank with more demanding, delicate, smaller plants. Slower growing plants also tend to be more susceptible overall than faster growing plants, which adapt more quickly to changes in the tank environment. A holistic approach to light, CO2 and fertiliser is necessary to grow plants well and consistently - this website has pages on the requirements of individual plants and how to adjust each variable.

Plant care plays a large part in the overall maintenance of the aquarium. All leaves age and deteriorate over time. Plants tend to put more energy and resources into defending new growth, leaving older leaves behind. This is especially true when the plant is short of growth resources. This means that a growing aquarium will always have a certain amount of decaying older leaves and old growth. This can lead to overcrowding, and decaying old growth often attracts algae. Old growth must therefore be regularly removed. This is done by cutting off and discarding older leaves, especially those that have already attracted algae to attach themselves. To rejuvenate growth on stem plants, it is necessary to periodically uproot the entire plant, cut off and replant the robust, fresh top part of the plant, and remove and discard the older, rooted bottom part.

 To read on how to keep a tank algae free through removing old growth, readthis article on solving algae issues without algicides. 

Lobelia cardinalis adapting after being transplanted. The plant defends the newer leaves against algae, while older leaves are discarded and attacked by algae. Trim away older leaves that are affected by algae to make room for new leaves to form.

Regularly replanting the fresh tops of stem plants while discarding the older, less robust bottoms will rejuvenate the plants and keep them free of algae.

Other traces of organic waste will remain in the water column and organic debris will accumulate on the substrate layer over time. These will also cause algae growth if not regularly removed. Water changing the 2hr Way aims at removing organic debris and algae spores that settle on the substrate layer of the tank. Doing this method greatly reduces algae spawning rate in the tank.