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. 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, especially if the tank has not been cycled before. Major but invisible changes occur in the tank upon submergence. The tank's microbial community needs time to mature; most hobbyists are familiar with ammonia cycling and how long it takes. Just as the filter needs time to develop a microbial community, 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 reprogramme 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. We often find that our freshly planted tanks become algae-filled messes within a few weeks.

Diatoms are the most common, along with hair algae or even staghorn and BGA. Algae invasions can occur within days in high light tanks. In more moderately lit tanks they usually appear in the second week or so.

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 tank - 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. While hobbyists have been obsessed with correlating certain nutrients or parameters with algae growth, the far more common trigger for algae in most tanks is unhealthy/old leaves that plants have given up defending. Plants sacrifice old growth when they are under adaptive stress or when their needs are not being met.

This is why we always say in the 2hr Way that healthy plants are the best defence against algae.

Do not panic and make sudden changes

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. Plants need time and a stable environment to grow well. If plants cannot keep up with environmental changes, they will be constantly vulnerable to algae.

Cycling the tank before planting

Cycling the tank, Soil in and with filter on, but no plants or lights, for a week or more to allow the tank to mature a little can help avoid some algae problems and ease plant transition stress. Adding some old soil during set-up or using established filter media can also help speed up microbial growth.

Starting the tank with moderate light less than 100 umols of PAR at substrate also leads to less algae triggering. For folks intending to run very high light systems; you can always tune it up after the plants have grown in.

The key is to have good parameters from the start, so that there are no drastic changes after plants are added. This is achieved by having good CO2 levels and a good fertiliser regime - and then keeping the parameters constant as the tank matures. These two critical aspects should be planned and adjusted beforehand, not after plants have been added.

Adjust CO2 levels before adding plants. Many people still underestimate the effect of CO2. Plants use more carbon than all other nutrients combined. Good CO2 levels are the best thing a tank can do to ease the transitional stress of aquatic plants. It is the key to getting difficult species to survive. Avoid using drop checkers, they are inaccurate and lag terribly. Learn to use the 1 pH drop method to estimate CO2 levels.

No changes were made to the above tank in terms of dosing, CO2 injection levels or any other measurable parameter. No algicides were used either. Diatoms reproduce very quickly in new setups and algicides are not very effective against them unless the root causes are addressed.

Steps to take, now that algae has invaded  

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. Additionally, any organic detritus collecting on the substrate surface should be siphoned off during water changes.

Trimming off older leaves

When plants produce leaves, the new leaf is programmed to suit the current environmental conditions. Leaves have limited plasticity and ability to adapt once they have matured. Therefore, if the tank conditions change or the plant is transplanted to a new environment, the plant must produce new leaves that are adapted to the new environment. Old leaves may be shed when they no longer contribute much (especially if the new environment is markedly different from the previous one) and these leaves become a prime target for algae.

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. How do we know when to remove older leaves? Being heavily affected by algae is a good sign that plants have stopped using resources to maintain that particular leaf. Aim to remove leaves that are very heavily covered with algae or damaged with holes..

The same patch of Alternanthera reineckii before removing algae infested leaves (above) and after removing deteriorating leaves to make way for new growth (below). 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. We can vary the length to replant depending on how much of the stem/leaves are in good condition and also what height we want a particular shrub to start at. Stems can grow from as little as one inch for smaller species to 3 - 5 inches or so for larger species. This action fills the aquarium with algae-resistant new growth while removing old growth that attracts algae. The basic requirement is that your growth parameters are in place for the plants to have produced robust new foliage that you can replant.

This pruning does not have to be done every time we want to shorten taller stemmed 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. We replant more often if the plants are affected by algae or have old growth that is badly deteriorated and attracts algae.

In this tank, the Lobelia cardinalis bush had just been trimmed and not replanted. The other bushes have been topped and replanted several times to remove the old growth and replace it only with the fresh top.

Timeline showing the impact of regular trimming, topping and replanting across a month

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. Diatoms are persistent. However, there is no need to panic or take drastic measures; experience has shown that the tank will stabilise on its own as long as fresh plant mass dominates. 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. More plant mass and well-formed new leaves in all species. Although the diatoms look serious, the amount of adapted plant mass is increasing day by day. 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.

Update:

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.

An aquarium with a predominance of fresh, robust, algae-resistant foliage is the best defence against algae problems. Plant management has a greater effect on algae than parameters, and most aquarists focus too much on tweaking parameters when considering algae problems. Parameters are important when it comes to plant growth, but tweaking parameters alone rarely solves problems associated with plant husbandry.

For tanks with slower growth rates - plants take longer to produce adapted leaves and fresh new growth. This means that refreshing the tank with new growth can take some time. This is most noticeable in non CO2-injected tanks where growth rates are slow. Therefore, this method alone will take time to produce results in such tanks. 

For tanks with fast growth rates, such as CO2-injected high light tanks with high nutrient levels, this method will produce results very quickly as there is a lot of fresh new growth being produced. However, hobbyists running such tanks can find it tedious to keep up with the trimming and replanting. Rapid growth also means that the tank will become overcrowded more quickly - today's new growth will become tomorrow's old growth and the cycle will repeat itself endlessly.

In the long run, hobbyists should set growth rates at a pace they can manage. If you do not have time for pruning and replanting, slow down the growth parameters or choose slower growing species.