Green water: why aquarium water turns green and how it clears

What green water is

Green water is not dirt and not a film on the glass. It is a bloom of microscopic algae suspended in the water itself, dense enough to color it green and, within days, to cut visibility sharply. Aquasabi names the green algae Chlorella, Ankistrodesmus and Scenedesmus as the usual culprits, with other floating algae such as the flagellate Euglena sometimes involved ^[1]. Because the cells float freely, they do not settle on glass, decoration or equipment ^[1], which is what distinguishes green water from algae films.

In lakes and ponds, floating algae are part of the phytoplankton and the base of the food web. The US Environmental Protection Agency describes algae as natural and important primary producers, and notes that excessive blooms can cause low oxygen and other problems ^[6]. It also points out that green algae blooms and cyanobacteria blooms are often confused, and that green algae are not generally known to produce toxins, unlike some cyanobacteria ^[6]. If the “green water” is accompanied by blue-green slime and a strong odor, see cyanobacteria instead.

What triggers it

The sources point to three triggers:

• Light. Aquasabi lists strong sunlight and seasonal warming in spring and summer ^[1]. Tropica’s checklist asks whether the sun shines directly on the aquarium, or whether the light timer has failed ^[2].
• New tanks. Aquasabi notes that blooms often happen during the cycling phase, before plants are established, and that a well-planted, well-fed tank usually outcompetes them in time ^[1].
• Nutrients. Tropica says plankton algae are normally caused by too high nutrient concentrations in the water and often appear in new aquariums or after a new layout ^[4]. Aquasabi attributes blooms in fertilized planted tanks to nutrient imbalances ^[1], and Tropica’s care guide adds that overfeeding fish is the most common cause of algae problems in general ^[3].

What clears it

Water changes

Tropica recommends more frequent water changes, 30 % every second day for a period, especially for green algae ^[2], and says plankton algae often disappear by themselves after repeated changes ^[4]. A well-functioning filter with plenty of fine filter material can also clear the water ^[4]. Each change removes a fraction of the suspended cells; the water-change calculator shows that four 30 % changes remove about three-quarters of whatever is in the water, before any regrowth.

Blackout

Tropica’s specific advice for green water is a blackout: cover the aquarium in black plastic and keep it dark for two to three days ^[2]. Aquasabi’s longer description adds the precautions: block all light including side light, keep the tank well aerated, switch CO2 and fertilizers off, and do a large water change afterward because the dying algae pollute the water ^[5]. A blackout clears the bloom but not its cause ^[5].

UV clarifier

Aquasabi recommends a UV-C clarifier, connected to its own low-flow pump so that water passes slowly enough to be treated, and run for a few days until the bloom lets up, followed by a large water change ^[1]. It advises against running UV permanently in a planted tank because it may affect the complexing agents in some plant fertilizers ^[1]; the iron and chelates guide explains what those agents do.

Tropica is more reserved. It agrees that UV treatment is often required for a stubborn bloom, but does not recommend it as a standard aid, because UV light also kills useful bacteria ^[4]. The two positions meet in practice: neither recommends UV as standard, permanent equipment.

Preventing a repeat

Tropica’s care advice is to fight algae with water changes, less fertilizer and more fast-growing plants ^[3]. Fast stem plants such as water wisteria are recommended by Tropica for new tanks precisely because their rapid growth takes up nutrients and helps prevent algae ^[7]. Floating plants and a regular photoperiod on a working timer address the light side.