Green dust algae (GDA) : A focused study

Introduction

Green dust algae is a persistent problem for some tanks, while others can be completely free of it. It is a nightmare for tanks that persistently get it because it returns very quickly, often within a day or two.

Most green dust algae (GDA) manifests itself as a thin green film on the glass walls of the tank, in severe cases this can form within a few days, while in less severe cases the glass will need to be wiped down every week or two. It is most common in new setups, not in the first or second week after water and plants have been added, but usually in the third week or later. In many setups, if GDA is not severe, it will fade as the tank grows in and 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 causes for GDA

Triggers for algae are often due to more than one factor, so two tanks with similar parameters can have very different results.

However, it is perhaps easiest to start with some general triggers for GDA that are broadly applicable to most tanks:

In new tanks GDA can occur simply due to tank ecology being new. In matured tanks, high light combined with nitrogen availability in the water column (whether in the form of ammonia or nitrate) is the common factor. This is particularly common in new setups around the second or third week. GDA is rare in low light setups 

Strategies for dealing with GDA

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.

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 about 70% of the substrate area planted, plant dominance will set in and many types of algae will disappear on their own. Planting heavily at the start will allow the tank to reach this state 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).

For tanks containing plants that do not require much light, keeping light levels low is extremely effective in preventing GDA growth. This is how Iwagumis prevent their hardscape from becoming infested with GDA. Contrary to popular belief, carpet plants do not require high light levels to grow well.

There are a number of scenarios that can trigger GDA in mature tanks. The use of fertilisers containing high nitrogen levels when GDA spores are already present is a common cause. Poor plant health due to inadequate or unstable CO2/nutrients or even poor horticultural skills resulting in unhealthy plants is another cause. When plants are stressed, damaged or unhealthy, they release organic compounds and ammonia into the water, causing algae spores to bloom. In older setups this can sometimes be caused by high levels of organic matter building up in the filter - cleaning the overloaded filter will help.

High macro-nutrient levels (especially high nitrate/NO3 levels) when starting a new tank will also often cause persistent GDA problems. This is more common in tanks using EI (Estimative Index) dosing due to the extremely high NO3 levels. In some of these scenarios, GDA persists even after the tank has matured and grown in. However, the cure is quite simple - reducing nitrate levels to 5ppm and below over a sustained period of time will stop GDA multiplying. Existing spots of GDA on hard scape may still need to be cleaned manually if you want a sparkling tank quickly. Once GDA has been absent for a period of time, nutrient levels can be increased again if desired.

At the other end of the spectrum, tanks owned by aquascapers who enter hardscape focused competitions tend to be meticulously clean of GDA as they want to present the look of the hardscape for the competition date. These tanks also often have a lot of hardscape, but can often be sparsely planted with slow growing mosses, so the tank does not have the luxury of being fully planted  with plant dominance winning the algae battle for the tank. So how do you keep these tanks free of algae?

There are a number of measures that competitive aquascapers take to ensure that the tanks grow without GDA on the rocks and wood. Very frequent water changes for the first few weeks (2 to 3 times a week for the first few weeks), including siphoning of detritus from the substrate surface. This will remove organic waste, ammonia and algae spores. The other aspect is careful control of water quality. Macro-nutrients (NO3, PO4) in the water column are kept very low and the plants are mainly substrate fed. Keeping the water column very low in nitrates, at least in the early stages of a new tank, will greatly reduce the incidence of GDA.

These types of tanks can often experience GDA later, as the tank matures and grows in, if the nutrient dosing is too low and the substrate nutrients are depleted. This is often associated with overcrowding of plants and old growth.

Summary

In summary, when it comes to nutrient dosing, regular lean dosing is the best middle ground as far as GDA is concerned. Too lean or super-rich systems are more unstable in different ways.

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.

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. The unfortunate reality is that most people do not have optimised setups and may never have optimised setups due to cost or time constraints.

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.

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 keep it well planted and moderate nitrogen availability in the water column.