Green dust algae (GDA) : A focused study

Introduction

Green dust algae can be a persistent problem for some aquariums, while others never encounter it. For tanks that persistently get it, it is a nightmare because it returns very quickly, often within a day or two.

Green dust algae (GDA) usually appears as a thin green film on the glass walls of a tank. In severe cases, it can form within a few days. In less severe cases, the glass needs to be wiped down every week or two. GDA is most common in new setups, usually in the third week or later after water and plants have been added. In many cases, if the algae is not severe, it will fade as the tank matures.

Sometimes GDA can continue to spawn even when everything else in the tank seems to be growing well, parameters are perfectly normal and there are no other forms of algae in the tank. This is particularly frustrating.

Common triggers for GDA

Triggers for algae are often due to more than one factor, so two tanks with similar parameters can have very different results. This has led to exaggerated claims such as "GDA is never related to light as I have a very high light tank with no GDA" or that "GDA is not related to nutrients at all as I have high nitrate levels in my water column and no GDA". What is true for one tank may not be true for another due to differences in other variables.

As with most algae types, high light is a factor that accelerates algae growth. However, usually it takes more than just high light alone to trigger algae to spawn.

In new tanks GDA can occur simply due to tank ecology being new and unstable. Organic waste, spikes in nutrients/ammonia etc can all trigger algae spores to bloom. Using less light during the first month and giving time for the tank to mature can resolve it in many cases without further intervention. 

In matured tanks, high light combined with a spike in nitrogen availability in the water column (whether in the form of ammonia or nitrate) is a common trigger for GDA. If the nitrate levels are sustained at a stable level, many tanks with high plant mass can adjust over time, and reach a stable state where GDA does not spawn continuously.

GDA can also occur when plant stress signal algae spores to bloom. Plant stress can be triggered by a significant change in water quality, nutrient levels or CO2 availability. In such cases, it is common to see other algae types spawn as well, such as BBA or Staghorn algae on leaves. When plants are stressed, damaged or unhealthy, they leak organic compounds and metabolites into the water column, trigger algae spores to bloom. This is once again, exaggerated by high light levels. 

Strategies for dealing with GDA

Achieving tank maturity quickly for new tanks

For newer aquariums, it is important that the tank reaches maturity quickly. This can be speeded up by using mature filter media from another tank or starter cultures. Good flow and gas exchange will improve oxygen levels and help the microbes to multiply more quickly. Another adjustment that can be made for those with adjustable lighting systems is to start the tank at lower light levels (around 50 PAR at substrate level). Then increase the light levels as the tank matures and grows in. (after the first 6 weeks or so).

Larger, frequent water changes (2-3X a week) for new aquariums, with siphoning off debris on the substrate surface removes algae spores and organic waste. This makes a huge impact on preventing algae blooms in newly planted aquariums.

Winning with plant dominance

A more densely planted aquarium can often out-compete GDA in the long run through plant dominance. Green dust algae often disappears on its own as the aquarium becomes biologically mature and most of the available space is taken up by plants. With at least 75%+ of the substrate covered with matured plants, plant dominance will start to set in and many types of algae will disappear on their own. In aquariums with plant dominance, high nutrient/nitrate values in the water column does not trigger GDA to spawn. However, many tanks simply do not have enough plant mass to reach this state.

The more matured and fully grown in a planted aquarium is, the greater the chances of hitting plant dominance. Planting heavily at the start will allow the tank to reach this state more quickly.

Tank grown with APT Feast and APT. The combination of nitrogen leaching from the substrate combined with higher nitrogen levels in APTe means that Nitrate levels measure around 8-12ppm in this tank.

Light and nutrient control for sparsely planted tanks with no plant dominance

For tanks containing plants that do not require much light, keeping light levels low (40-60umols of PAR on substrate level) is extremely effective in preventing GDA growth. This is one approach Iwagumi layouts prevent their hardscape from becoming infested with GDA. Contrary to popular belief, carpet plants do not require high light levels to grow well as long as carbon availability (CO2 levels) are good.

Nutrient control is the other angle. In such aquariums, relying on a rich substrate to feed plants nitrogen/phosphorus and keeping the water column lean in nitrates prevents GDA from forming on glass and hardscape. For our Iwagumi layouts, the water column measures virtually 0ppm of nitrates, and we do not dose nitrate fertilizer in the water column. Aquasoil is used to supply nitrogen to the carpet. 

We grow out Iwagumi layouts with a combination of rich substrate (APT Feast) and lean water column dosing (APT1), coupled with lower light levels (50-60umols of PAR on substrate). This combination makes Iwagumi layouts easy to manage. 

A mix of strategies? What about a high light tank with low plant mass due to tank design?

For tanks that utilize a large amount of hardscape/decorative sand that reduces planting areas, it can be difficult to achieve high plant mass and plant dominance in the tank.

High light, low plant mass is a difficult combination to manage as it does not fit cleanly in either strategy outlined above. These tanks are more vulnerable to algae issues. What one can do is to minimize algae triggers as much as possible in such layouts.

• Using a rich substrate, lean water column - dose N/P into the water column if necessary, but only as much as it takes to keep plants healthy. 
• Using only as much light as necessary - and not more.
• Religious maintenance in the form of frequent, large water changes and keeping the tank clear from detritus, deteriorating leaves/old growth.
• Mosses and other plants such as Bucephalandra/Anubias can grow well on hardscape, with allows even a hardscape focused tank to have more plant mass.

Maintaining white/clean hardscape in a high light tank requires optimizing from various fronts.

Misconceptions and counter-productive thinking

Some common misconceptions that people face when trying to solve GDA problems:

The first is the idea that because "fertilisers do not cause algae", all nutrients can be liberally dosed at high levels and all algae problems can be easily solved by adjusting CO2 or plant mass, or by better maintenance alone. This only works for aquariums which can achieve plant dominance - you need a very fully grown in aquarium for this to happen. (75%+ of the substrate covered with plants).

High nutrient levels do not cause algae only if the tank is healthy, well stocked with high density, healthy plants. Tanks that are vulnerable in one way or another (sparsely planted tanks, non-cycled tanks, tanks with damaged or unhealthy plants) will have worse algae problems if nitrates/phosphates are very high. Having elevated nitrate levels in a tank that is already suffering from GDA is one of the main scenarios that makes it extremely persistent in some setups. 

A much better approach when a tank is currently experiencing a GDA problem is to dose enough to keep the plants healthy, but to keep the NO3/PO4 levels in the water low (less than 5ppm) until the GDA problem is resolved.

At the other end of the spectrum, severely under dosing macros (mainly NO3) for fear of GDA in the first place can also cause GDA if the plants become unhealthy. Whatever dosing regime you choose, you need to give at least enough to keep the plants healthy. Rich soils allow very low water column doses for long periods, but even rich soils will become depleted over time and either soil enrichment or increased water column doses will be required.

Are hot tanks doomed to have GDA?

Some people blame high tank temperatures as the cause of GDA, but high temperatures are also just an accelerant. We run the tank below at 31 degrees Celsius to demonstrate that you can defeat GDA even in a very warm tank, as long as you follow one of the strategies outlined above.

This tank is dosed with APT 3, and with APT Feast substrate, running at 31 degrees celsius with a heater. However, it is very densely grown in, and matured enough that plant dominance has set in.

Click here for information on how to solve algae problems without algicides.

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

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