Green Water (Chlorella): Why Your Tank Looks Like Pea Soup
Description
Green water is the "alga" that does not look like an alga: it is millions of microscopic Chlorella cells floating and the water turns cloudy, a pea-soup green, sometimes within a single week. It does not stick to the glass or leaves, because there is nothing to see except the colour.
It does not harm fish directly, but it blocks the view, takes light away from the plants and, when the cells die all at once, uses up oxygen. A normal filter does not hold it back, because the cells are smaller than the pores of the filter media.
It is typical of sunny tanks, new set-ups or tanks with few plants, and it clears up well once you cut off what feeds it: light and surplus nutrients.
Why they appear
Too much light or direct sun
Direct sun on the glass, or a photoperiod of ten hours or more with a strong light, gives the suspended algae more than enough energy. It is the most frequent cause when the outbreak appears suddenly.
Treatments
Cut the light
- Move the tank away from the window or cover the glass that gets the sun.
- Cut the photoperiod to six or seven hours.
- If the outbreak is heavy, switch the light off and cover the tank completely for three or four days, then do a water change.
Surplus nutrients
High nitrate and phosphate, from overfeeding, too many fish, infrequent water changes or fertilising without testing, feed the cells. If the plants do not use them, the alga does.
Treatments
Bring the nutrients down
- Change a quarter of the water every day for three or four days and then once a week.
- Feed less and remove whatever is left.
- Test the water and stop fertilising until the values are under control.
UV steriliser
A UV steriliser connected to the filter passes the water by a lamp that destroys the suspended cells. It is the quickest and cleanest fix: the water clears in a few days, although it does not replace fixing the cause.
Few plants and an immature filter
In a new tank, or one with few fast-growing plants, nothing competes with the alga for nutrients, and the filter does not yet hold the bacteria that keep the water stable.
Treatments
Plant more and give the filter time
- Add fast-growing or floating plants, which use up nitrate and give shade.
- Let the filter mature and do not over-clean it meanwhile.
- To clear the water faster, filter through very fine floss that traps the cells; replace it as soon as it clogs.
Sources
- GBIFGBIF Secretariat (2026). GBIF Backbone Taxonomy. Checklist dataset. https://doi.org/10.15468/39omei. Consultado vía GBIF.org, 2026-08. Chlorella.
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.