How do you control algae without algicides?

The top picture is taken after the first week of a new set-up, diatoms are just starting to form and many of the leaves have a large amount of algae on them. This is pretty common for most newly set up planted tanks.

The same tank is below after one month, with no use of algicide and exactly the same plants as above; no new plants have been added. There have been no parameter changes either - so what has been done?

The first few weeks are the toughest

A planted aquarium goes through many changes in the first few weeks of a new set-up. Major but invisible changes occur in the tank upon submergence. The tank's microbial community needs time to mature; the substrate ecology needs to change from dry to submerged form, with a different population of microbial life adapted to underwater soils. These microbes perform important functions such as breaking down organic waste and oxidising harmful compounds such as ammonia. Larger microbes are also responsible for consuming algae. Read more about algae and aquarium maturity here.

Newly planted plants go through a period of adaptation stress as they reprogram their proteins and enzymes to make best use of the resources in their new environment. Some of these changes are obvious, such as in plants grown under water adapting to submerged conditions by growing underwater form leaves that look markedly different from their emersed leaves. Other changes are less visible - such as the optimisation of RuBisCO, an enzyme involved in the first major step of carbon fixation, to match the carbon dioxide levels of the new environment. The plants' immune systems are weakened by the stress of the transition, and algae opportunistically bloom and attach themselves to the plants, feeding on metabolites that leak from the leaf margins. Plants are most vulnerable to algae in newly set up tanks.

Diatoms are the most common, along with hair algae or even staghorn and BGA. 

Close-up of the tank from above in the second week. A mixture of staghorn, green spot algae and diatoms are invading the tank.

However, if we look closely, there is a lot of fresh growth that is relatively free of algae, while it is the old growth that is heavily affected by algae. This is an important observation; algae do not affect all areas of the tanks equally - they tend to attack the older growth on plants, leaving the new, fresh foliage relatively unaffected. Why is this?

Robustly growing plants produce a range of antimicrobial chemicals - various alkaloids, terpenes, phenolics - as a defence against pathogens and microbes. Plants will prioritise the defence of valuable new leaves first, as these newest leaves are optimised and adapted to the current environment and contribute the most. Leaves have limited plasticity, so older leaves have limited ability to adapt to new environments. At the same time, the plant can draw reserve energy and nutrients from older leaves to fund the growth of new leaves. The greater the change in environment, the more we see this at work. As the plant stops defending the older leaves, metabolites leak from the leaf margins and attract algae to spawn.

While the exact mechanism of plant defences and algae triggering can be complicated, the useful observation is this: Old or unhealthy leaves attract algae, while robust, fresh new leaves are resistant to algae. A tank that can maximize robust fresh growth will be an algae-resistant tank.

Parameters need to be stable for plants to adapt

When faced with a sudden onslaught of algae, the first instinct of most hobbyists is to make drastic changes to the tank parameters, thinking that the algae is caused by too much or too little nutrients, CO2 or some other parameter. This is the worst course of action. When plants are trying to adapt to a new environment, constantly changing the environment is a recipe for disaster. Tweak CO2, nutrients quickly when the tank is first planted, but keep things stable there after.

Trim away old growth that is infected with algae

As the plants adapt to the environment, the new growth should be in good shape and relatively free of algae, with algae mostly affecting older leaves. We will aim to remove the worst of the old growth, while making room for the new leaves to grow. This will be done through a combination of pruning, uprooting and replanting the tops. 

Cutting off these older leaves allows new leaves to grow and take their place. It removes a potential home for algae while making room for new growth. 

Removing decaying leaves to make way for new, algae-free growth makes a huge difference to the health of the plant and the overall resilience of the aquarium to algae.

Replanting tops, discarding bottoms

For faster growing stem plants, we have the option of discarding the whole of the older stem and replanting the fresh top. This requires us to uproot the entire stem plant and replant only the top 3 inches or so. This action fills the aquarium with algae-resistant new growth while removing old growth that attracts algae. 

This does not have to be done every time we want to shorten taller stem plants. If the bottoms are relatively healthy, we can do a straight pruning instead. This means that the tops are simply cut off and discarded, leaving the rooted part to grow back. 

For a comprehensive guide on how to maintain healthy stem plants long term, click here.

Additionally, any organic detritus collecting on the substrate surface should be siphoned off during water changes. This removes organic waste and algae spores that trigger algae blooms in high light tanks.

Repeated cycles of trimming away old growth and replanting new growth:

Week 2 - New growth can now be seen on most plants. However, there is a lot of poor quality older growth that needs to be removed. Diatom build-up is skimmed off during water changes to prevent it smothering the plants.

3rd week; carpet filling in more. Ludwigia arcuata colours appear. Further trimming of older leaves and replanting of stems on the right side of the tank. During the weekly water changes, substrate detritus and excess diatoms are removed as much as possible by hand.

4th week; as adapted plant mass takes over, diatoms decrease. Shrimp and fish are introduced. Not only does the new growth look good, but the plants are defending the older leaves to be free of algae. This is a sign that most plants are fully adapted to the current tank environment.

Week 5; As the algae subside, the focus can be shifted to shaping the bushes into the desired shape. At this point the light can be turned up to produce deeper colours and more animals can be added to the tank. Topping and replanting is still done on the right side bushes to bring out better shapes for those species.

These are a couple of pictures taken a few weeks later; the Rotala blood red bush has been trimmed and the Ludwigia arcuata bush has been replanted. As long as the plants (including the HC carpet) are rejuvenated over time, the tank can be kept in good shape. Anyone seeing the tank as it is today would never imagine the algae cycle it went through in the early stages:

Conclusions

The continuous action of removing older, algae-attracting foliage/growth and leaving algae-resistant new foliage will transform the tank from an algae-infested mess to a relatively clean and fresh looking set-up within a month without the use of algicides. This concept is most applicable to new aquarium setups where plants are going through a significant period of transitional stress. However, it applies as a general approach to all planted aquariums. Old growth needs to be periodically removed and refreshed with new growth.

The tank above is fully grown in less than 2 months.

Click here for information on how to stabilize a new planted tank quickly.

Click here for information on how to do proper water changes for a planted tank.