Green Spot Algae (Coleochaete): The Hard Dots on the Glass
Description
Green spot algae is recognised by its bright green, hard, round dots, one to three millimetres across, scattered over the front glass and on slow leaves such as those of anubias and cryptocorynes. They do not wipe off with a cloth: they are stuck on and only come off with a blade or a card.
It belongs to the genus Coleochaete and is one of those that appear once the tank is established: a recent set-up does not bring it, a nutrient imbalance does, almost always a phosphate level that is too low.
It is not dangerous, but it spoils the look of the tank and, left to grow, covers whole leaves. It pays to scrape the glass and tackle the cause at the same time.
Why they appear
Phosphate too low
When phosphate falls close to zero, because of reverse-osmosis water, plants that use it all up or a filter media that holds it back, green spot algae gains ground. It is the most common cause.
Treatments
Raise the phosphate
- Test the phosphate: below about 0.5 mg/l the alga has the upper hand.
- Add a phosphate fertiliser in small doses until you are around that value.
- Test again after a few days and adjust.
More light than the CO2 and nutrients can support
Strong light or a long photoperiod, without the CO2 and fertiliser to back it up, leaves the plants behind, and the alga takes advantage of that gap.
Treatments
Adjust the light
- Cut the photoperiod to seven or eight hours.
- Lower the light intensity or raise the lamp.
- If there is CO2, check that it reaches all the plants.
Old, slow-growing leaves
Leaves that have been on the plant for months, above all on anubias, cryptocorynes and ferns, accumulate the alga because they renew slowly and cannot defend themselves.
Treatments
Remove, scrape and let grazers work
- Trim heavily covered leaves: they will not recover.
- Scrape the glass with a scraper or an old card.
- Nerite snails graze green spot better than most algae eaters.
Sources
- GBIFGBIF Secretariat (2026). GBIF Backbone Taxonomy. Checklist dataset. https://doi.org/10.15468/39omei. Consultado vía GBIF.org, 2026-08. Coleochaete.
Taxonomy verified as ACCEPTED unless stated otherwise.
The scientific name is a genus (spp.): the whole genus is catalogued because its species cannot be told apart by eye in an aquarium.
The causes of appearance and the treatments have no citable source: GBIF supports the taxonomy, not the management of algae in aquariums. A source for algae is still to be agreed.