Algae Control 101: How to prevent algae in an aquarium?

Introduction

Although algae removal techniques can be effective, they should not be the default method of keeping a tank algae free. The cleanest tanks are those that are algae resistant through good design. This article will look at the key considerations when designing an algae-resistant tank. This will include equipment considerations from the time the tank is set up, how much hardscape is used and what percentage of the available substrate is planted. The strategies for keeping a fully planted "Dutch style" tank from getting algae are quite different from keeping a sparsely planted Iwagumi setup from getting algae.

We will look at the main factors that determine algae presence in a tank and look at some tank examples on top of that.

Two very different styles of tank, yet each can keep algae at bay if designed properly. Let us start with the first factor - how healthy plants are naturally algae resistant.

1.Healthy plants are algae resistant, unhealthy plants attract algae

Healthy plants that receive sufficient nutrients, light and carbon in a favourable environment produce antimicrobial chemicals that prevent pathogens and algae from attacking the plant. Unhealthy plants that are deprived of what they need will divert energy into defending the youngest leaves, but older growth will quickly deteriorate. You will often see that when algae attack a plant, the older leaves are affected first, while the newer leaves remain bright and clean.

The tops of stems growing close to the top of the tank receive a huge amount of light. Light levels at this level are in excess of 400 umols PAR. Notice how these plants remain free of algae despite the very high light levels. If light alone were the cause of algae, then the top parts of the plants would be most affected, but this is not the case. Healthy plants are naturally resistant to algae.

Unhealthy plants that are deficient in nutrients tend to have pale growth, which often attracts algae. This tank is probably receiving much less light than the one above, but poor plant health brings algae problems, even with less light.

Maintaining good plant health is a priority

Growing healthy plants may seem like a simple concept, but there are many aspects to it. Basically, all plants need a regular source of nutrients, carbon and light to grow well. This site has detailed pages for each of the main requirements: Nutrients, Carbon and Light. We call them the 3 pillars of plant growth.

But these are not the only variables that can affect plant growth - plant husbandry, overall biological maturity and tank stability all play a part in ensuring plants grow well. Plant selection must also be taken into account. Some species require more light or nutrients than others, some do much better with CO2 injection, while some species can be grown easily in low-tech tanks. Some plants prefer soft water while others prefer hard water.

Folks that have done well with some types of plants may struggle with other types. Stem plants need their own horticulture techniques described on this page. 

Larger, more robust plants (swords, lotus, crinum) are more resistant to algae than smaller plants (carpets & mosses). Fast, aggressive-growing plants (water wisteria, large-stemmed plants) are more resistant than slow-growing plants (Java fern, Anubias, Alternanthera reineckii). Due to tank design and flow patterns, some tanks may have algae confined to a specific area of the tank. Planting larger, more robust plants in these areas can solve persistent algae problems.

Difficult species = harder job to maintain good plant health = harder time with algae

Plants with more demanding growth requirements are also, by definition, more prone to algae, as many may not be able to meet their needs adequately (causing them to grow in a stressed state). Having a larger percentage of hardy plants mixed with a few difficult species will make the tank easier to maintain overall. If you are just starting out, avoid buying too many difficult species at once; build up your collection slowly and give yourself time to understand the needs of each plant. If you have persistent algae problems with only one species or type of plant, changing to something more robust is an easy way to get around the problem. Sometimes changing the location of a difficult plant in the tank will also help; it may need a bit more water flow or light that is available in a better spot.

Mixing plants that prefer high light (Hygrophila sp Chai) with plants that prefer more subdued light (Anubias white petite) makes for good social media posts, but is tricky to manage. Using higher light will cause algae to spawn on the slower growing Anubias, while using lower light will cause the Hygrophila sp Chai to not be as well-formed and lose its older leaves more easily. 

Adaptation to environmental changes

Any time growth parameters are drastically changed, plants must instead divert energy to reprogramming their enzymes to optimise for the new conditions. For example, if an aquatic plant is moved from a tank with high levels of carbon dioxide (CO2) to a tank with low levels of CO2, the plant would divert more energy to proteins and enzymes responsible for CO2 uptake to compensate for the drop in CO2 levels. As the plant undergoes this adaptation, it prioritises critical functions, leaving less energy to produce defensive antimicrobial chemicals.

Under stress, plants will generally stop defending the older, less valuable leaves first. Algae will then spawn opportunistically on the affected (usually older) leaves. If the plant recovers in time, the affected leaves can often recover. Often, however, the plant will choose to divert energy into the production of new leaves at the expense of the oldest leaves. Old leaves do not heal and must be cut away to make way for new growth. Preventing plants from going through such periods of stress is important in keeping the aquarium free of algae.

In this poorly designed low-tech tank, plants that do not receive enough nutrients and carbon to maintain their existing mass will constantly divert energy to maintaining new growth (white arrows), while old growth will always look bad because the plant does not have enough energy to defend it against algae (red arrows). If the basic parameters of the tank are not sufficient to support good plant health, this symptom of algae on older leaves will never go away. The lack of carbon in many low-tech tanks is the reason why the lower stems of stem plants are always bare.

This Samolus parviflorus red has recently been transplanted into this tank. The plant has produced new leaves adapted to the current tank conditions - these new leaves are healthy and remain free of algae (white arrows above). The older leaves contain machinery optimised for the previous tank and the plant is abandoning them; the decaying old leaves attract algae.

Just 20 days later, the plant has grown a new set of leaves adapted to the new tank conditions. As long as the conditions in the new tank are kept stable, this healthy plant will have no problem maintaining the health and quality of its older leaves.

Removing older, deteriorating growth makes room for new growth. Simply removing algae with algicides does not remove the underlying causes - deteriorating old growth or plant stress factors.

In this example, the nutrient levels in the tank were increased significantly within a short period of time. (Residual NO3 levels in the tank rose from 1ppm to 8ppm within a few days.) The increased nitrogen levels caused the plant to reprogramme itself to increase growth rates to take advantage of the newly available nitrogen. As this happens, waste proteins and metabolites are expelled from the plant and algae spawn on the leaf surfaces. Although growth conditions have become more favourable, the short-term reprogramming of the plant's internal machinery causes the algae to bloom. This will disappear as the plant regains its health, but sometimes persistent forms of algae can appear. In such cases, a dose of algicide is a quick cure.

The key lesson from above is that stability is very important. Most common plants are adaptable within a wide range of nutrient and even CO2 levels. Rapidly switching between high and low levels of nutrients or CO2 and triggering adaptation shock in plants is what triggers algae.

How much plant mass?

Now that we know that healthy plants deter algae, how much of the tank do we need to plant for the tank to have a good defence against algae? If the substrate is 80-100% planted, healthy plant mass will keep algae at bay. The main focus of the aquarist in such tanks is to maintain plant health by providing adequate nutrients/CO2/light and preventing overcrowding by regular trimming and replanting.

Head here to see an example where we cure an algae filled tank without algicides (just by using plant growth).

Due to the algae resistant nature of fully planted layouts, it allows the usage of higher lighting and nutrient levels to grow difficult plant species that have higher light and nutrient requirements.

As plant mass increases, so does the demand for CO2 and nutrients - the aquarist's main task is to ensure that the plants' needs are met. Competition for nutrients can be increased in fully-planted, fast-growing setups, and under-fertilising such tanks will result in weak plants, deterioration of older growth and algae.

At 2hr Aquarist, our plant focused tanks are naturally algae resistant due to the high plant mass. However, plants also compete with each other for space. Plant husbandry is critical to maintaining a densely planted tank that does not suffer from over-crowding. All tanks above are CO2 injected and dosed with APT fertilizers.

Summary

• Healthy plants are algae resistant, unhealthy plants attract algae.

• Having regular fertilization, and a strategy to get adequate carbon into the tank is essential for good plant health.

• Plants can be growing, yet be under stress or be unhealthy.

• Old, deteriorating leaves attract algae and should be cut off.

• Stability matters. Plants are adaptable, but adaptation stress causes algae. 

• Over-crowding leads to poor plant health and quicker decaying old growth. Prune consistently; a tank fully filled with new growth is extremely algae resistant.

• Tanks that are 80-100% planted are much more algae resistant compared to lightly planted tanks.

2.Light and nutrient control

We discussed fully planted tanks using plant mass to deter algae in the section above. What about very sparsely planted tanks? The Iwagumi above is less than half planted, and only with delicate ground cover, no less. The strategy for keeping algae at bay in such setups is therefore quite different from the fully planted 'Dutch style' examples seen previously.

Light and nutrients, together with CO2, are the 3 main factors influencing plant growth in aquatic environments. The stronger the light and the more nutrients available for plants to take up, the faster the potential growth rates. However, strong lighting combined with organic waste or stressed plants will trigger algal spores to bloom.

Once these spores have bloomed, the fully formed algae will grow faster if light and nutrient levels are high. In high plant mass tanks, which are naturally resistant to algae, higher levels of light and nutrients in the water column will not trigger algae because the plant mass will out-compete the algae and deny them a place to spawn. In tanks with low plant mass, plant dominance is not there to help, so instead we need to minimise algae triggers by limiting light levels and limiting certain nutrients in the water column.

How low is "low light"?

Head here to read about what is PAR.

Below 60umols PAR, the incidence of inducing algae is greatly reduced compared to tanks running above 100umols PAR. For hardscape tanks, running between 40-60 umols of PAR at substrate level will allow the hardscape to remain relatively free of algae. This level of PAR is sufficient to grow most types of green plants, including the carpeting plants used for Iwagumi setups. Contrary to popular belief, carpeting plants do not require high light levels to propagate if CO2 levels are sufficient.

CO2 injection is effective even in low light tanks. CO2 injection with low light levels is one of the most stable combinations available in the hobby.

The Iwagumi above uses just 50 umols of PAR on the substrate to grow a carpet of dwarf hairgrass. Carpets are not high light plants, contrary to popular belief. Species such as Dwarf Hair Grass and Monte Carlo will thrive in 40-60 umols of PAR as long as CO2 levels are reasonable (20ppm +). Running a tank at 50 umols of PAR vs 100 umols of PAR is a world of difference if you do not have the defence against algae that a densely planted tank provides.

This 4ft aquascape has carpet plants at substrate level mixed with sand and hardscape. 40-60umols PAR is enough light to allow the plants to grow and spread, yet low enough to minimise algae.

Light intensity guide

This is a guide to the appropriate light intensity as measured on the surface of the substrate (more details of how to measure this in the light section). Using the minimum amount of light required to grow the style of tank that you want is a smart way to reduce algae.

• 15-30umols of PAR: Low light plants such as ferns, mosses, Cryptocoryne, Anubias, Bucephalandra species grow well. If you are growing purely shade plants, it is smart to stay in this range.

• 40-60umols of PAR: Can grow any common commercially available green aquatic plant (when coupled with optimal CO2 levels). However, this gives little allowance for shading issues in crowded tanks. Most nature style and Iwagumi style tanks that do not require high density do well at this range. 

• 100-150umols of PAR: Colored stems will display better coloration and carpets spread very fast as light levels increase, at the risk of more algae issues. Tank cleanliness and plant health must be maintained at optimal conditions to prevent run-away algae. Allows for increased planting density for light demanding plants. Most experienced aquarists can do well in this range with regular upkeep. Tanks should be more densely planted to avoid algae.

• 200umols of PAR & above: Marginal gains to plant growth form and coloration starts to be outweighed by large potential instability of algae outbreaks. Might be useful inducing stronger coloration in shorter stems close to substrate; and for farming purposes. Should only be used by aquarists used to maintaining already algae-free tanks.

Proserpinaca palustris is one of the few plants that see improvement in coloration/form with very high light levels. Most colored/red plants can be grown well without such excessive light.

Light duration

• Minimum of 5-6 hours for new tanks that want to minimize triggering algae.

• 7-10 hours is common for average, stable tanks.

• 12+ hours; marginal gains to plant growth rates, increased chances of algae incidence. Suggest using moonlighting/lower powered display lighting if extended lighting hours is desired for viewing purposes.

Having an adjustable light system gives flexibility. Changing the number of hours as well as intensity, by using dimmers or hanging kits, can change outcomes considerably.

Power timers plug into standard power outlets and allow easy programming of when lights turn on/off.

Moving Nitrogen & Phosphorus into the substrate

In addition to limiting light levels, the other thing we can do to reduce the occurrence of trigger algae is to move nitrogen and phosphorus into the substrate. What does this mean? It means that the rooted plants are supplied with nitrogen (N) and phosphorus (P) through the use of a rich aquasoil substrate, rather than through water column dosing. This allows us to dose a water column fertiliser with reduced or no N.

In Iwagumi setups, using a rich aquasoil instead of sand allows one to root feed the plants nutrients instead of relying on heavy water column dosing. 

One can most certainly grow in an Iwagumi using more light and nutrients in the water column. While the carpet will spread faster in a high light, high nutrient tank,  the tendency is that green algae will also grow on the hardscape readily.

In this Iwagumi layout, we use around 40 umols of PAR on the substrate level. Additionally, we root feed the plants nitrogen and phosphate purely from the substrate, by using a water column fertilizer that is free from nitrogen and phosphate (APT 1). 

For tanks where there is some hardscape usage, but also a mix of coloured plants, a balance needs to be struck. In this tank we use 120 umols of PAR on the substrate. This is high enough to give good colouration on the red plants and good density on the background stems, but difficult to keep the rocks bright white. We also use a rich aquasoil substrate to move N into the substrate so we can dose less N in the water column. We dose an N limiting fertiliser into the water column (APT 3); nitrate levels for this tank bottom out at 0ppm 24 hours after dosing.

In this setup we run 200umols of PAR at substrate level to get deeper reds and denser growth. The high plant mass in the tank protects the environment from algae - note how most of the substrate is covered by plants. In such tanks we are more concerned about nutrient depletion - regular dosing of a water column fertiliser and the use of a rich Aquasoil substrate prevents nutrient levels from bottoming out.

Summary

• Use the least amount of light necessary to minimize algae triggers.

• If you are not growing colored plants, staying between 40-60umols of PAR makes the tank much easier to manage.

• If you are running very high PAR (>150umols at substrate), you need very high plant mass for an algae resistant tank.

• Root feeding plants N/P by using a rich aquasoil allows one to have less N/P in the water column. Having less N/P in the water column reduces algae incidence in sparsely planted tanks.

• Using a rich aquasoil prevents nutrients from bottoming out in a densely planted tank.

3.Organic waste & Biological Maturity of the tank

These two concepts go hand in hand. In the same way that decaying plant matter triggers algal blooms, organic waste (whether from animals or plants) is one of the main triggers for algal blooms. Efficient bio-filtration and a mature tank environment will lead to faster breakdown of organic waste and ammonia, minimising the trigger time for algae. This means that an environment that promotes microbial activity, allowing bacteria to break down waste products faster and more efficiently, has a major impact on the tank environment.

Many microbes also consume algae directly as food. Diatoms, which are common in new setups, disappear once the tank has matured without any other intervention. Newbies panic when they see diatoms appear in a new setup, while experienced aquarists just snooze and wait for it disappear. 

In this video, and in many other videos on the channel, you can observe microbes consuming algae.

Sustaining the delicate microbial ecosystem

Microbes are delicate and heavy use of harsh chemicals (such as algicides) will affect them. This is why many aquarists get caught up in the never-ending cycle of killing algae - having new algae - looking for new cures, while never having a sustained period of algae-free aquaria. Prevention is better than cure - if you are going to use algicides, apply them to problem areas at an early stage rather than bombarding the whole tank.

Most microbes benefit from flow and good oxygen levels, so good flow through the tank is beneficial. Good oxygen levels can come from either a lot of plants photosynthesising or good gaseous exchange; this means a clean water surface and exchange of oxygen-rich surface water with layers deeper in the tank. Microbes live on surfaces, including those of plants. Substrates that have a large surface area and are bio-active, such as Aquasoil, provide a much better habitat for the microbial population than pea gravel. Having a good amount of filter media also provides a home for microbial communities.

For a new tank, adding mulm and/or squeezing detritus from a matured filter is the fastest way to seed a new tank with microbial life. 

We use surface skimmers on all our tanks to keep the water surface clear. Surface skimmer inlets also draw oxygen-rich surface water down into the filter, where microbes help to digest organic waste. While these tanks look neat and tidy, great care is taken not to upset the ecology of the tank.

Dealing with organic waste

While dissolved organic waste is easily removed by water changes, excessive mulm on the substrate can also trigger algae growth. The idea of a soil substrate getting dirty is an extremely strange concept for newer hobbyists - isn't soil just a collection of dirt? The organic waste that accumulates on the surface of Aquasoil is very different from the components that make up Aquasoil. This is better understood through the idea of composting - many gardeners compost their kitchen waste before adding it to their gardens. This creates a biologically stable medium compared to the overly labile conditions that would be created by adding raw kitchen waste directly to the garden soil. Similarly, Aquasoil is made up of weathered organic matter and clay, which is much more biologically stable than the undigested organic waste that accumulates on the substrate surface over time.

While the accumulation of organic waste on the aquasoil substrate does provide some nutrients, it does so at the expense of being a significant trigger for algae. It is much cleaner to remove the labile organic waste layer and fertilize the tank through pure elemental salts - this results in less algae triggers.  

Water changes should not just focus on replacing tank water - aquarists should also use this chance to siphon away detritus that has build up at the substrate layer. More detailed steps on how to maintain aquasoil substrates long term here.

To do it; hover a siphon over the surface of the substrate while kicking up surface detritus with a turkey baster. For weekly cleaning, you should not aim to disturb more than just the top layer of the substrate. For more details on how we do it, see here.

This tank is about 2 years old when the picture was taken. Large regular water changes, with regular siphoning of substrate detritus keeps it looking clean. This tank runs on APT 3 and APT Jazz. Observant viewers will note how clean the aquasoil looks despite the age of the tank. This observation can be applied to most tanks that run very high light levels - you will seldom see a pristine algae free tank with a very dirty substrate zone. 

An example of a lightly planted tank by Dou Mok. The plant selection is also one that is most vulnerable to algae issues (Ferns, mosses and anubias). The fact that they remain algae free is a testament to the high maintenance standard of the aquarist. Such designs seem deceptively simple, but in fact take a lot of expertise to manage to maturity. Vacuuming substrate debris, removing dead and decaying leaves and pruning over-crowded growth are all important actions in tank maintenance. 

Shrimps, snails and other micro-fauna keep the tank clean by breaking down organic debris. This sand path with mixed carpets is more than a year old; fortnightly light vacuuming of the surface keeps it clean.

Summary

• Regular water changes (30-80% weekly), coupled with siphoning of substrate detritus makes a huge impact on organic waste levels.

• Removal of old/decaying growth. This requires topping and replanting - cutting off and replanting the top portion of stem plants, while discarding the deteriorating bottoms. Details here.

• Servicing clogged filters. Depending on your bio-load this could be done as frequently as every fortnight or up to every 3 months. Using a filter with an easy to remove pre-filter chamber that does not require dismantling of the whole filter is an easy way to make maintenance easy.

• Maintain high oxygen levels in the tank through sufficient gaseous exchange mechanisms. Higher oxygen levels is important for bacteria activity.

• Using organic/soil-based substrates or appropriately sized sand (2-3mm) in the tank rather than inert large sized gravel. Increased porosity allows better bacteria colonization.

• Filter flow rate of 5X to 10X of tank size. 1000L/hr filter on a 200L tank for example.

• Maintaining stable water parameters long term. This also means avoiding spikes of toxic elements (heavy metal content in water for example).

• For a new tank, adding mulm and/or squeezing detritus from a matured filter is the fastest way to seed a new tank with microbial life. 

Putting it all together, FAQs

Hardscape heavy or plant heavy?

Tanks that are more than 80% planted can rely on plant dominance to immunise the tank against algae. These tanks can use more light and fertiliser than lightly planted tanks. Picky species that require more light/fertiliser to grow are almost always kept in such plant-heavy tanks. Plant-heavy tanks require much more maintenance - trimming and replanting are among the most time-consuming tasks in the long-term maintenance of a planted tank.

Tanks with a lot of hardscape need lower light levels and less nutrients in the water column to be naturally free of algae. However, it is also possible to carry out intensive maintenance on such tanks to keep them free of algae - many competitive aquascapers change the water on a daily basis, which allows them to run higher light levels on hardscape-heavy tanks while avoiding algae.

Competition aquascaper Matthew manes grows out his competition tank above using APT 3; the same fertilizer we use in our plant focused tanks. However, he does very regular water changes to keep algae at bay in his hardscape focused layout.

Are you choosing to run high light just for 1 or 2 species in a tank?

Sometimes we raise the light levels in the tank just to accommodate 1 or 2 picky species that require more light to grow well. One should question whether the trade off is really worth it. Sometimes just adding 1 or 2 troublesome species increase tank maintenance tremendously. 

In this tank, all plants except the Rotala blood red SG bush at the back would be fine growing at low light levels (<60umols of PAR). We bumped up light levels to 120umols just to maintain the red stem plant bush in good color.The red stems are quite key to the design of the overall layout in this case. We do more frequent water changes to maintain the sand front in the higher light levels.

Looking for a low maintenance tank? 

A low light tank that is fully planted with easy, slower growing plants is the easiest tank to maintain algae free. Slower growing plants also means lower maintenance with regards to trimming and other horticulture work. Avoid stem plants and non-green plants. 

Another approach is to let hardscape carry the weight of the design of the tank, so that one does not need to take care of so many plants.

This low light setup at the 2hr Aquarist gallery makes use of slow growers such as Bucephalandra and Cryptocoryne species. Measuring just 30-40 umols of PAR at the substrate level, this low light tank is a breeze to maintain. The plus side is that the tank always looks presentable - there are no stem plants that require time to recover from trimming, no waiting for a ground carpet to grow in. Low maintenance does not mean no maintenance though - the tank gets weekly water changes, and substrate detritus is siphoned away weekly as well. Water column fertilizer is dosed a couple of times a week. 

Will CO2 injection solve all my problems?

Many folks running low tech tanks are overly ambitious with their plant selection. Depending on how the low tech tank is set up, a lack of access to carbon is often the biggest issue for many plant species, and unhealthy plants are an algae magnet. Being overly ambitious with plant selection is the biggest reason many plants do not do well in low tech tanks. Choosing species that are well suited for low tech tanks greatly improve success rates when CO2 injection is not available. Head to this page for a selection of plants that are proven to do well in non CO2 injected tanks.

CO2 injection, when available, boost growth rates for most aquatic plant species by 7x to 10x. Plants use more Carbon as part of their growth than all other nutrients combined. CO2 injection allows tanks to out-grow their algae problems. When plants grow faster, it also allows the aquarist to learn faster, so CO2 is not only an accelerator for plant growth, it also accelerates the learning cycle for aquarists. However, CO2 does not act alone. It needs to be paired with a good fertilizer dosing regime, and ultimately good maintenance to be effective in a tank.

In this low tech tank at the 2hr Aquarist gallery, substrate PAR measures just around 20-30umols. A selection of hardly low tech plant species are chosen. 

Here are links for further reading on the following topics:

1. How to clear algae without algicides

2. How to use algicides to clear algae

3. Detailed steps to tank cycling

4. What nutrients do aquarium plants needs for growth?

5. Section on CO2 for planted tank

6. Section on aquarium plants care guides 

7. For detailed steps to control specific algae, click here.

8. More detailed steps on how to maintain aquasoil substrates here.