Green dust algae is easy to remove and almost impossible to keep away, which is exactly what makes it so frustrating. Wipe the glass and the aquarium looks perfect. Four days later it is back. The reason sits in how the algae reproduces, and once you understand that, both of the methods that actually work make sense.
What is green dust algae?
Green dust algae, or GDA, is a soft fine green film that coats aquarium glass in an even layer. It wipes off with almost no effort, which is the key identifier. In severe cases it extends onto plants and hardscape.
It poses no threat to fish, shrimp or snails, and no real threat to plants. It is purely a matter of appearance, but a comprehensive one. An aquarium with GDA looks neglected even when everything else about it is perfect.
A biologically immature aquarium running heavy water column fertilisation, with nitrate above 20ppm, under a high output LED at full power. Five days apart.
Green dust, green spot or green water?
If yours is hard and spotted rather than a soft film, read how to get rid of green spot algae instead. The cause is the opposite and so is the fix.
Why does it keep coming back?
This is the part most guides skip, and it explains everything else.
GDA has a two stage life cycle. The film you see on the glass is the settled, attached stage. As that film matures it releases free swimming zoospores into the water column. Those spores circulate, settle, and start a new film.
So when you scrape the glass part way through that cycle, you do not remove GDA from the aquarium. You break the film apart and disperse it through the water, seeding every surface at once. That is why the tank often looks worse a week after a thorough clean than it did before you touched it.
Scraping an immature film does not export it. It disperses the spores through the water column, which seeds every surface at once.
Both of the methods below work around this. One removes the spores from the aquarium entirely. The other lets the cycle finish before you intervene.
What causes green dust algae?
- A biologically immature aquarium. By far the most common factor. GDA typically appears between weeks two and eight after setup.
- High light on a lightly planted tank. Full intensity over young plants that cannot yet use it.
- Excess water column nutrients relative to plant uptake. Heavy dosing into an aquarium whose plants have not grown in.
- Excess organics. Aquasoil breaking down, detritus accumulating in the substrate, a heavy fish load.
Worth being honest about this: GDA in a new high tech setup is close to unavoidable. It is a maturity problem more than a parameter problem. Established aquariums do get it, usually when aquasoil starts breaking down or waste builds up in the substrate, but the classic case is a brand new tank with strong light and generous dosing.
Method 1: Remove it and export the spores
The faster method, and the right choice if you need the aquarium presentable now.
90% to 95%. This is what physically exports the free swimming zoospores from the aquarium rather than just moving them around inside it.
With the water level down, work on the glass above the remaining water. Dispose of each towel after a single wipe. Do not rinse it, do not reuse it, and do not let what you have wiped fall into the water below.
If plants are coated too, remove them, clean or trim them out of the tank, then replant. Leaving them in place gives the film somewhere to survive the whole exercise.
Use dechlorinated, temperature matched water. Repeat once or twice a week for two to three weeks in severe cases. Each round removes more of the spore population than it leaves behind, and the cycle eventually breaks.
At this volume of water changing, every drop needs treating. AquaLabs Vitality neutralises chlorine and chloramine within seconds, so there is no waiting before the tank goes back together.
Method 2: Let the cycle complete
The patient method, and one many experienced aquascapers prefer.
Do not touch the glass at all for three to four weeks. Let the film mature fully. Once it has, it releases its spores and then sloughs away from the glass in sheets, often within a day or two. At that point do one very large water change, wipe the glass down once, and clean your filter.
When it works, GDA frequently does not return, because you have allowed the cycle to complete rather than repeatedly interrupting it part way through.
The honest caveats: this is less reproducible than method 1, results vary between aquariums, and you are looking at a green tank for a month. If the aquarium sits in a living room, method 1 is the more practical choice.
What not to do
Common mistakes
- Scraping the glass with the filter running. The single most common mistake and the one that actively makes GDA worse. You have released the spores and then left them circulating in the tank.
- A small water change after cleaning. Removes a fraction of the spores and leaves the rest. If you are going to clean the glass, commit to the large change.
- Reaching for an algaecide. GDA is a fast recolonising free swimming film, so a spot treatment has nothing lasting to act on. Physical export and parameter correction are what work here.
- Increasing light because the plants look sad. Understandable and counterproductive. The plants are struggling because the aquarium is immature, and more light feeds the algae first.
How to stop it returning
Removal buys you a clean aquarium. These adjustments are what keep it clean.
Reduce your lighting
Cut the photoperiod to four or five hours and reduce intensity to 40% to 60% if your light is dimmable. Hold it there. Only start increasing once GDA has stayed away for two full weeks and the plants have grown in properly, and then add no more than 30 minutes per week.
This is the change people resist most and the one that helps most.
Move to lean dosing
Reducing water column nitrate significantly slows GDA.
AquaLabs Lean Grow is formulated for exactly this, supplying what the plants need from the water column while leaving the substrate to do the heavy lifting on nitrate.
One important limit. Lean dosing is only appropriate in aquariums with aquasoil. Aquasoil holds a nutrient reserve the roots can draw on, which is what makes running a lean water column viable. In an aquarium with inert substrate such as sand or gravel, the water column is your plants' only nutrient source, and nitrate needs to stay above 10ppm. Going lean in an inert tank will starve the plants and hand you a worse algae problem than the one you started with.
Reduce organics
- Siphon detritus from open substrate, but avoid stirring aquasoil, which releases a pulse of nutrients
- Service the filter properly, rinsing mechanical media in old tank water
- Remove trimmings and decaying leaves rather than letting them break down
- A surface skimmer removes the organic film at the water surface and improves gas exchange at the same time
AquaLabs Pro Skim clears the protein film from the water surface, which both cuts the organic load feeding GDA and lets the surface exchange gas properly.
Give it time
Most aquariums clear GDA permanently between weeks eight and twelve, as the biofilter matures and the plants grow in enough to outcompete it. If you are in week three of a new tank, a good part of what you are dealing with is simply the tank being new.
What eats green dust algae?
- Bristlenose catfish. The most effective grazer for GDA by a clear margin. Juveniles work harder than adults because they need the food to grow. The trade-off is that bristlenose produce a substantial waste load, and in a small aquarium that added organic load can feed the problem you are trying to solve. Best suited to tanks of 150 litres and up.
- Otocinclus. Nearly as effective, far smaller waste load, and completely shrimp safe. The better choice for anything under 150 litres. Keep them in groups of six or more.
- Nerite snails. They will graze GDA, but rarely fast enough to keep pace with its growth rate.
None of these will out-eat an active GDA bloom. They keep a corrected aquarium clean rather than curing one that is not.
The short version
- GDA returns because scraping disperses its free swimming spores instead of removing them
- Either export the spores with a 90% water change, or leave the film alone and let its cycle finish
- Never scrape the glass with the filter running
- Wipe with disposable paper towel above the waterline and bin each one after a single pass
- Cut the photoperiod to four or five hours and drop intensity to 40 to 60%
- Move to lean dosing, but only in an aquarium with aquasoil
- Inert substrate needs nitrate above 10ppm, so lean dosing there will backfire
- Reduce organics with substrate siphoning, filter servicing and surface skimming
- Bristlenose in larger tanks, otocinclus in smaller ones
- Most new aquariums clear it permanently between weeks eight and twelve
GDA is the most persistent looking algae you will meet and one of the least serious. It is a new aquarium finding its feet, and the aquarium wins in the end.