Algae guide
Green Spot Algae: Hard Circles on Glass and How to Prevent Them
Last updated: May 2026
- What it is: Hard, circular green discs that bond directly to glass and slow-growing leaves - not a soft film you can wipe away.
- Root cause: Low phosphate is the counterintuitive trigger. GSA thrives when phosphate drops below roughly 0.05 mg/L, which happens most often in heavily planted tanks with aggressive fertiliser regimes.
- Removal: A razor blade scraper on glass; nothing else works reliably. Spots on leaves are permanent - trim the leaf.
- Best biological control: Nerite snails (Neritina spp.) are the only algae eater that makes a real dent on glass GSA. One snail per 10-15 litres is a practical stocking rate.
- Prevention: Keep phosphate at 0.5-2 mg/L, maintain consistent CO₂ if you run it, and don't let light hours creep above 8 per day without strong plant growth to match.
What green spot algae actually is
Green spot algae (GSA) is caused primarily by Coleochaete spp., a group of encrusting green algae that calcify as they mature. That calcification is the whole problem - it's what makes GSA feel like it's been glued on rather than grown on. Run your fingernail across a patch and you'll feel individual raised discs, each anywhere from 1 mm to 5 mm across. Early spots are easy to miss because they're translucent; by the time they're obviously green, they've already hardened.
GSA grows slowly. You won't wake up to a tank covered overnight. Instead it accumulates over weeks on the front glass, equipment casings, and the broad leaves of slow-growing plants like Anubias and Microsorum. That slow growth is partly why people underestimate it - it doesn't look like a crisis until it's genuinely widespread.
The phosphate paradox: why clean tanks get it worst
This is the thing most guides either bury or get wrong entirely. GSA is not primarily a high-nutrient problem. It's a low-phosphate problem.
Phosphate (PO₄) is a macronutrient that plants consume in significant quantities. In a well-planted tank with regular dosing, phosphate can crash to near zero between water changes. When it does, Coleochaete outcompetes your plants for the scraps because it can grow on almost nothing. The tanks most prone to persistent GSA are Dutch-style aquascapes and high-tech planted tanks where the keeper is proud of how pristine their water looks - they've accidentally created ideal GSA conditions.
The target phosphate range to suppress GSA is roughly 0.5-2 mg/L. Below 0.1 mg/L and you're in GSA territory. Above 2 mg/L you'll start feeding other algae types. Test phosphate specifically - it won't show up on a general API master test kit, so pick up a dedicated PO₄ test or use a digital meter if you're dosing precisely.
The fertiliser industry has done a good job convincing hobbyists that "balanced" all-in-one liquid fertilisers solve everything. They don't. Many popular all-in-ones keep nitrate healthy but dose phosphate conservatively to avoid complaints about green water. If GSA is your recurring problem, check your PO₄ separately before assuming your dosing is adequate.
Other causes that stack up alongside low phosphate
Phosphate depletion is the primary driver, but GSA rarely appears from a single cause. These factors compound the problem:
Too much light, not enough plant growth
Running the lights 10-12 hours a day to make the tank look nice is one of the most common setups in the hobby, and it's also a recipe for algae. GSA benefits from extended photoperiods. If your plants aren't growing fast enough to consume the light energy you're providing, algae will use it instead. Eight hours is enough for almost any planted tank. If you're running high-output LED fixtures, 6-7 hours of full intensity with a ramp-up and ramp-down is often better.
Inconsistent CO₂
In CO₂-injected tanks, fluctuating carbon dioxide levels cause plant metabolism to stall. When plants stop efficiently taking up nutrients - including phosphate - the levels in the water column swing erratically. GSA exploits those gaps. If you inject CO₂, keep it stable: 20-30 mg/L measured by drop checker (lime green colour), and never let it fluctuate more than a few mg/L across the day.
Slow water circulation
Patches of low flow create nutrient dead zones on the glass surface. GSA establishes itself more readily on glass with poor turnover past it. Aim for total tank volume turnover of 5-10× per hour through your filter and any powerheads, distributed to eliminate still spots.
Reducing water changes to "preserve" nutrients when GSA appears. This logic seems sound but usually makes things worse - old water accumulates organic compounds that suppress plant health while phosphate remains locked up in unavailable forms. Consistent water changes of 25-30% weekly are part of the solution, not the problem. See our guide on how often to do water changes for more on this.
How to remove green spot algae
On glass: razor blade only
There is no chemical shortcut and no algaecide worth using in a planted tank. The only reliable removal method for GSA on glass is a metal razor blade scraper. The plastic-blade alternatives sold cheaply in most fish shops don't generate enough force to break the bond. Get a proper stainless scraper - the kind with replaceable blades - and work at a low angle with firm pressure. The spots release cleanly once you get under the edge.
Do this before a water change so you can vacuum out the loosened material. GSA fragments left in the water will resettle and reattach, though they take a while to re-establish.
Never use a metal razor blade on acrylic tanks - it will scratch irreparably. On acrylic, you're limited to hard plastic scrapers or melamine foam, which means accepting that GSA on acrylic is much harder to remove fully. This is a genuine reason to choose glass over acrylic if GSA-prone conditions are likely in your setup.
On leaves: trim, don't scrape
GSA spots on Anubias leaves, Java fern fronds, or any other slow grower are permanent. Scraping damages the leaf tissue and often leaves a worse cosmetic result than the spot itself. Trim the affected leaves at the petiole and let new growth replace them. This is frustrating when you have a large mature Anubias barteri with 30 spotted leaves, but there's no better option.
Fast-growing plants like Vallisneria, Hygrophila, and stem plants rarely show GSA because their leaves are replaced too quickly for the algae to calcify.
Nerite snails: the only biological control worth relying on
Nerite snails (Neritina natalensis, Vittina semiconica, and related species) are the best biological tool for GSA management. They're the only common aquarium algae eater capable of rasping calcified green spot off glass consistently. Otocinclus, plecos, and shrimp will browse softer algae but largely ignore hardened GSA.
For meaningful control, stock one Nerite per 10-15 litres. In a 60-litre tank, four to six snails will visibly reduce glass GSA within a week. They won't eliminate a heavy infestation by themselves - you still need to scrape it down first - but they'll keep glass cleaner between manual cleans once conditions improve.
The practical stocking rate for meaningful GSA control on aquarium glass.
One genuine downside: Nerites lay small white calcified eggs on hard surfaces - glass, decorations, equipment. The eggs are harmless and won't hatch in freshwater (Nerite larvae require brackish conditions to develop), but they're difficult to remove and some people find them ugly. If your tank has visible front glass as a display surface, position your Nerites toward the sides and back where egg deposits are less visible.
Nerite snails are escape artists. Any gap in your lid - filter inlet notches, cable holes, even a 1 cm opening - is enough for them to leave the tank overnight. Seal gaps with foam weatherstripping or mesh cut to size. A snail that dries out on your floor for more than a few hours is almost certainly dead.
Fixing your phosphate levels without destabilising the tank
If testing confirms phosphate is genuinely low (under 0.5 mg/L), you need to raise it carefully. Slamming in a large dose of phosphate at once can shift pH - particularly in softer water - and cause a temporary algae bloom from the sudden nutrient spike. The right approach is gradual.
Powdered monopotassium phosphate (KH₂PO₄) is the standard option for planted tank keepers. It's cheap, precise, and doesn't add unwanted ions. Dose small amounts - start at 0.1 mg/L increments - test after 24 hours, and repeat until you're in the 0.5-1 mg/L range. If you run an auto-doser, micro-dosing daily is far more stable than weekly manual additions.
If you're using an all-in-one fertiliser and still showing low phosphate, either the product is underweight on PO₄ for your plant load, or your plants are consuming it faster than the label dosing accounts for. Supplement with separate PO₄ rather than doubling the all-in-one - the latter will likely push nitrate too high before it fixes the phosphate shortage.
For a full breakdown of how nutrients interact with tank stability, the guide on aquarium water parameters covers the relationships in more detail.
What to do if GSA keeps coming back
Recurring GSA after you've addressed phosphate usually points to one of three unresolved issues: light duration is still too long, plant mass is insufficient for your tank volume, or CO₂ is inconsistent (in injected tanks). Work through them systematically rather than chasing individual parameters.
A practical diagnostic sequence: reduce photoperiod to 7 hours for two weeks. If GSA slows but doesn't stop, test phosphate again. If phosphate is now adequate, add more fast-growing plants to increase nutrient uptake and compete more aggressively with the algae. If you're on CO₂, log your drop checker readings at the start and end of the light period - a significant colour shift indicates instability that needs fixing at the regulator or diffuser.
Logging your test results consistently makes this much easier to track over time. App-aquatic lets you log water test results, set reminders for fertiliser doses, and spot trends in parameters that are hard to catch when you're testing sporadically.
Track your parameters and get care reminders with App-aquatic.
Get the free appIs green spot algae harmful to fish or plants?
Not directly to fish. On plants, heavy GSA coverage on slow-growing leaves like Anubias can reduce photosynthesis over time, but it won't kill a healthy plant quickly. The main impact is cosmetic - spotted glass and leaves look poor in any display tank.
Why do I have GSA if I've never dosed fertilisers?
Unfertilised tanks often have very low phosphate precisely because there's no supplemental source. Fish waste provides some nutrients, but heavily filtered, lightly stocked tanks can easily drop below the threshold where plants compete effectively. Light duration and intensity relative to your plant load matters as much as fertiliser in this scenario.
Will hydrogen peroxide spot treatments work on GSA?
It's unreliable on GSA specifically. Hydrogen peroxide works reasonably well on softer algae types - BBA and hair algae in particular - but the calcified structure of GSA protects the cells from oxidative treatment. You'll get better results spending that effort on a razor blade and addressing the underlying phosphate issue.
How many Nerite snails do I need for a 100-litre tank?
Six to ten is a practical range for a 100-litre tank with active GSA. Start with six and see how much ground they cover in the first two weeks. They won't overbreed in freshwater, so stocking a few extra isn't a long-term problem the way it would be with livebearers or pest snails.
Can GSA appear in a brand new tank?
Yes, and it's common in new tanks for two reasons: plants are still establishing and not yet consuming nutrients efficiently, and lighting is often set too long by new keepers who want to watch their tank. During the first few weeks, keep your photoperiod to 6-7 hours and scrape glass weekly until the tank matures. Make sure you've properly cycled the tank first - here's how the nitrogen cycle works if you need a refresher.