Green spot algae is the hardest algae in the hobby to scrape off and the easiest to prevent. It is also the one algae you beat by feeding your aquarium more rather than less, which is why so many people spend months making it worse.
What is green spot algae?
Green spot algae, or GSA, appears as small hard circular green dots between half a millimetre and two millimetres across. It bonds tightly to whatever it grows on and will not wipe, rub or brush away. On glass you need a blade. On a plant leaf it is effectively permanent.
Green spot algae on Anubias. Slow growing plants hold the same leaf long enough for GSA to bond into the surface.
It turns up on aquarium glass, on hardscape, and on the leaves of slow growing plants such as Anubias, Bucephalandra and Java fern. Fast growing stem plants almost never get it, because their leaves do not live long enough for it to establish.
Green spot or green dust?
These two get mixed up constantly, and they have opposite causes, so thirty seconds spent checking will save you weeks.
If yours is a soft film that comes away on a cloth, read how to get rid of green dust algae instead.
What causes green spot algae?
Low phosphate
This is the primary cause and the one to fix first. When phosphate in the water column runs to zero, your plants cannot grow efficiently and GSA takes the opening.
The most common reason for zero phosphate in an aquarium that is being fertilised properly is the substrate. Most aquasoils adsorb very large quantities of phosphate, and they are at their most absorbent in the first months after setup. You can be dosing a complete fertiliser and still test zero, because the soil is stripping it out of the water faster than you are adding it.
Water column phosphate in a newly planted aquasoil aquarium at a constant dosing rate. The soil strips almost everything you add for the first months, then releases its grip as its capacity fills.
This is exactly why GSA is so common on Anubias and Bucephalandra in the first three to six months of a new aquascape, and why it often fades on its own later without anyone changing anything.
The other routes to zero phosphate:
- Running a fertiliser that contains no phosphate at all
- RO or rainwater with a remineraliser that adds none
- Phosphate removing filter media, which has no place in a planted aquarium
- Simply underdosing relative to your plant mass and light
Low or unstable CO2
Secondary, but real. Plants that cannot access enough carbon cannot use the nutrients available to them, which leaves those nutrients to the algae. If phosphate is corrected and GSA persists, CO2 is the next thing to check. The same 1.0 pH drop by lights on that prevents black beard algae applies here.
A photoperiod longer than the plants need
Eight or nine hours over an aquarium of Anubias and moss gives the algae more light than the plants can use. Six to seven hours suits most low tech setups.
Old leaves
Worth saying plainly. Some GSA on a two year old Anubias leaf is normal, even in a well run aquarium. A leaf that has sat under light for eighteen months will accumulate something. This is not a sign that anything is wrong.
What phosphate should you be targeting in the water column?
Worth being clear about what these numbers are. They are a concentration your aquarium should hold, which is what your test kit reads. They are not a dose. How much fertiliser it takes to reach and hold them depends on your plant mass, your light, your CO2 and how hungry your substrate is.
That last point is the one people miss. Adding CO2 does not only make your plants grow faster, it makes them consume everything else faster too. An aquarium that held a comfortable phosphate level without CO2 can run itself down to zero within days of injection starting, and green spot algae is often the first sign of it.
You cannot correct a nutrient you are not measuring, and phosphate is not included in most standard test kits. The AquaLabs Phosphate Test Kit reads the range that matters in a planted aquarium, so you can see whether you are sitting at target or sitting at zero.
Do not be alarmed by a high phosphate reading. Phosphate does not cause algae in a planted aquarium. This is one of the most persistent myths in the hobby and it has created far more algae than it has ever prevented, by pushing people to starve their plants.
Which fertiliser fixes it
If GSA is your problem, you need a formulation that carries real phosphate. Here is what each of ours adds per pump.
Phosphate added by one pump (2.0ml) per 30 litres.
Note that Lean Grow is lean on nitrate, not phosphate. It carries more than double the phosphate of Easy Grow, which makes it a strong choice for a hardscape led CO2 aquarium fighting green spot algae. Easy Grow Lite adds no phosphate at all by design, so it is the one formulation to avoid if GSA is your problem.
For green spot algae in a CO2 injected aquarium, AquaLabs Lean Grow or Max Grow both supply phosphate at a level that shuts it down. Without CO2, Easy Grow is the right fit. Not sure which? See which fertiliser does my aquarium need.
How to remove what is already there
A steel razor blade is the only thing that reliably removes GSA from glass. Work at a shallow angle and keep the blade flat. Use a plastic blade on acrylic, since steel will scratch it. Melamine sponges and standard algae magnets will not touch it.
A stainless or brass brush works well. For pieces you can lift out, scrubbing them dry outside the aquarium is faster and far more thorough.
GSA bonds into the leaf surface and scrubbing will only damage the plant. Either leave it, since it is harmless, or cut the worst leaves off at the petiole. Anubias and Bucephalandra regrow from the rhizome readily.
Once phosphate is corrected, all new growth comes in clean. You are waiting on new leaves, not on the old ones recovering.
Scraping treats the symptom. If the aquarium is still running at zero phosphate, the glass will be spotted again within a fortnight.
What eats green spot algae?
Almost nothing, and this is where a lot of advice online goes wrong. GSA is hard and firmly bonded, so it needs a grazer with a rasping mouthpart strong enough to work it off the surface. Most algae eaters are built for soft films and simply cannot do it.
- Nerite snails. Genuinely effective, and the only species worth adding specifically for the job.
- Ramshorn, bladder and pond snails. Moderately helpful and slow, but they do make progress.
- Otocinclus, bristlenose, Siamese algae eaters and shrimp. None of these will remove established green spot algae. They eat soft films and will graze around it.
Snails only keep pace once the root cause is fixed. Add nerites to an aquarium running zero phosphate and the algae will outgrow them.
What not to do
Common mistakes
- Cutting nutrients to starve it. The exact opposite of the fix. GSA is caused by phosphate scarcity, so reducing your dosing accelerates it.
- Running phosphate removing media. It has no place in a planted aquarium and it guarantees the problem.
- Scrubbing plant leaves. You will destroy the leaf and the spots will still be there.
- Panicking about a high phosphate test. High phosphate does not cause algae in a planted tank. Zero phosphate does.
- Expecting old leaves to recover. They will not. Correct the cause and let new growth replace them.
The short version
- GSA is caused by phosphate running to zero, not by phosphate being high
- Fresh aquasoil strips phosphate hard for the first three to six months, which is when GSA is most likely
- Target 1 to 4ppm PO4 in a high tech aquarium, 1 to 2ppm for steadier growth or without CO2
- Zero is never a target, and CO2 lifts phosphate demand so an injected tank hits zero faster
- Test phosphate rather than guessing, since it is missing from most standard kits
- Lean Grow and Max Grow carry the most phosphate. Easy Grow Lite carries none
- Check CO2 next if phosphate is corrected and GSA persists
- Razor blade on glass, stiff brush on hardscape, never scrub a leaf
- Cut the worst leaves and let clean new growth replace them
- Nerite snails are the only grazer that genuinely removes it
Green spot algae is the most cosmetically irritating and least harmful algae you will meet. Get phosphate off zero and it stops appearing on anything new.