Why Your Tank Has Turned Green, and How to Clear It

In this guide

A tank that has gone from clear to a hazy, pea soup green almost overnight is dealing with a free floating algae bloom, most often a microscopic organism called euglena, and while it looks alarming, it is also one of the more reliably fixable problems in the hobby once you understand what is actually driving it.

Not the same problem as algae on the glass

Green water is a free floating bloom that tints the entire body of water, not the green film or spots that build up on glass, decorations or plant leaves, which is a separate and much more common form of algae with its own fix, usually a scrub with an algae pad or magnetic cleaner. Wiping the glass will not touch true green water, since the algae causing it is suspended throughout the tank rather than growing on a surface, which is why a magnetic cleaner alone never seems to fix a genuinely green tank.

What causes it

Green water needs two things to take hold together, light and nutrients, and it is rarely just one or the other. Excess light usually comes from a tank sitting in or near a sunny window, a light left on for too many hours a day, or a light that is simply too strong for the tank. The nutrient side comes mostly from nitrate and phosphate, which build up from overfeeding, an overstocked tank, or decaying plant matter, and can also arrive through tap water that is itself high in these compounds.

The fastest fix: a UV sterilizer

Running the tank water through a UV sterilizer is widely considered the quickest and most reliable way to clear a green water bloom, often within two to three days. The unit passes water past a UV bulb that damages the algae cells' DNA so they stop reproducing, and it is safe for fish and plants when installed correctly, typically on the outlet side of a canister or internal filter. Once installed, it can be left running to prevent blooms from recurring rather than only being used as an emergency fix.

The no-equipment fix: a blackout

If buying a UV unit is not practical, covering the tank completely with black cloth or bin liners for three to five days, uncovering only briefly each day to feed, starves the algae of the light it needs to survive. Fish cope with this without any issue, since they need oxygen and food, not light, during the blackout period. Live plants can suffer if kept in complete darkness for more than about three days, so this method suits a fish-only or lightly planted tank better than a heavily aquascaped one. Follow the blackout with a generous water change, around 50 percent, to clear out the dead algae cells rather than leaving them to decay in the tank.

Addressing the underlying cause

• Move the tank away from direct sunlight, or fit a curtain or film over the nearest window, since many Indian homes have a balcony-facing or window-adjacent tank catching several hours of strong afternoon sun
• Put the tank light on a timer set to six to eight hours a day rather than leaving it on inconsistently
• Cut back feeding to what fish finish in two to three minutes, and remove anything left over promptly
• Add fast growing live plants, which compete with algae for the same nitrate and phosphate and generally win that competition once established
• Continue regular water changes throughout treatment, since they dilute both the nutrients feeding the bloom and the algae cells themselves

A word on chemical clarifiers

Liquid water clarifiers can clump algae cells together so a filter traps them more easily, and they have their place as a supporting step, but they treat the symptom rather than the cause. Pairing one with a genuine fix to the light or nutrient imbalance behind the bloom gives a far more lasting result than reaching for a bottle alone.

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