Green spot algae: what it is and what sources say causes it
Short answer
Green spot algae are hard, flat green dots that grow into crusts on glass and on long-lived leaves such as Anubias; Aquasabi says they probably mostly belong to the green-algal genus Coleochaete [1, 2]. Aquasabi links them to strong light and to phosphate that is either too low or too high [1], while EI's author reports never seeing them at high phosphate [3]. They do not wipe off; a blade scraper and snails are the usual control.
What it is
“Green spot algae” is an aquarium name for a growth form rather than a species: small, flat, hard green dots that cling tightly to surfaces and, if conditions suit them, widen into continuous crusts [1]. They appear on aquarium glass, equipment and decoration, and on plant leaves, especially the long-lived, tough leaves of Anubias [1].
Aquasabi’s wiki says they probably mostly belong to the green algae of the genus Coleochaete, which have a plate-like, multicellular body [1]. In the GBIF Backbone Taxonomy, Coleochaete is a genus in the order Coleochaetales, within the charophyte green algae [2]. No source cited here identifies the organism in any particular aquarium; the attribution is a general one.
How to tell it from other green films
Aquasabi distinguishes spot algae from other green coatings by two traits: spot algae start as distinct dots, and they are always hard to remove, while many other green films lack the dot stage and some can be wiped off easily [1]. If a fingernail or a soft sponge clears it, it is probably something else.
Conditions associated with it
Sources link spot algae to two things, and agree on only one of them.
Light. Aquasabi says spot algae are frequently found in strongly lit tanks [1].
Phosphate. Here the sources point in different directions:
| Source | Claim |
|---|---|
| Aquasabi wiki | Spot algae usually appear if phosphate is too low or too high; suggested range 0.1–1 mg/L PO4; above about 3 mg/L, reduce it [1] |
| EI (Tom Barr) | Green spot algae “has never been an issue” when high phosphate levels were maintained, even under high light with Anubias; he dosed close to 3 ppm PO4 week after week [3] |
Both are practitioners’ observations rather than controlled studies, and they are compatible only if low phosphate is the common trigger. If spot algae appear while phosphate is at the bottom of a test kit’s scale, both sources would suggest raising it; above 3 mg/L, they would disagree.
Slow leaves. Tropica explains why the same plants keep collecting it: Anubias leaves live for years, which gives slow-growing algae time to establish [4]. The same reasoning applies to other slow plants with long-lived leaves.
Removal and control
- Scrape. Aquasabi recommends a blade cleaner for glass, as part of weekly maintenance [1]. Removing it also makes regrowth easy to spot.
- Algae-eating snails. Aquasabi names nerite snails and Clithon species as the most efficient grazers of these hard coats in its experience [1]. Tropica’s general algae advice is to use more algae eaters, and more than one kind [6].
- Leaves. Badly coated old leaves can be trimmed off.
- Maintenance. Tropica’s general measures against algae apply here as well: water changes, not overdosing fertilizer, more fast-growing plants [5].
Phosphate can be checked against the arithmetic in the fertilizer dosing calculator, and the water-change calculator shows how quickly a change brings a high level down.
Questions
Is green spot algae a sign of too much or too little phosphate?
Sources disagree. Aquasabi says spot algae usually appear when phosphate is too low or too high, and suggests 0.1–1 mg/L [1]. EI's author says green spot algae were never an issue when he kept phosphate high, up to about 3 mg/L, even under high light with Anubias [3].
Calculators for this
- Dry fertilizer dosingConvert grams of KNO3, KH2PO4, K2SO4, Epsom salt or a trace mix into mg/L for your tank, or work back from a target ppm. Every molar mass is shown.
- Water-change dilutionWhat a water change does to nitrate, phosphate, TDS or any dissolved level, the change needed to hit a target, and where weekly dosing settles.
Plants mentioned
- Anubias barteri var. nana: grower data, planting and namesrhizome, epiphyte / foreground / midgroundLight: LowCO2: LowEasy
- Bucephalandra pygmaea 'Bukit Kelam': grower data and namesrhizomeLight: LowCO2: LowEasy
- Staurogyne repens: grower data, planting and its Rio Cristalino originstem, foreground / midgroundLight: LowCO2: LowEasy
Related guides
- FertilizersNitrogen, phosphorus and potassium in planted tanksWhat N, P and K do in plants, the forms plants take up, how aquarium dosing expresses them, and what growers and dosing methods say about each.
- CO2 & lightPhotoperiod: how many hours a planted tank's light should runWhat growers and retailers recommend for daily lighting hours, how photoperiod and intensity add up to the daily light dose, and when shorter days are advised.
- Algae & problemsBlack beard algae: the red algae behind it and what sources adviseBlack beard (brush) algae are red algae despite their color. The likely genera, the alcohol test, conditions linked to it, and the control methods sources describe.
- Algae & problemsHair, thread and fuzz algae: telling them apart and what helpsGreen filamentous algae in planted tanks: how thread, hair, fuzz and Cladophora differ, the light, nitrate and CO2 triggers growers name, and the grower remedies.
Sources
- Green spot algae (GSA). Aquasabi (aquascaping wiki). Hard green spots widening to coats; on glass, equipment, decoration, long-lived Anubias leaves; strongly lit tanks; probably mostly Coleochaete; phosphate too low or too high; nerite snails; blade cleaner. Accessed October 12, 2026.
- Coleochaete Brébisson, 1844 (GBIF Backbone Taxonomy record 2638383). GBIF Secretariat. Genus in Charophyta, Coleochaetales, Coleochaetaceae. Accessed October 12, 2026.
- The Estimative Index of Dosing, or No Need for Test Kits. Tom Barr, The Barr Report (2005). Green spot algae never an issue when high PO4 levels were maintained, even under high light with Anubias; dosed up to almost 3 ppm PO4. Accessed October 12, 2026.
- Anubias barteri nana (plant details, 101PCS). Tropica Aquarium Plants. Slow growth and leaves that live for several years give slow-growing algae the chance to establish. Accessed October 12, 2026.
- Care. Tropica Aquarium Plants. Algae signs include accumulations on the glass; reduce with water changes, less fertilizer, more fast-growing plants and algae eaters. Accessed October 12, 2026.
- Algae control. Tropica Aquarium Plants. More algae eaters, more frequent water changes, light control, blackout. Accessed October 12, 2026.