Green water is algae that lives in the water itself
Unlike the algae film you scrub off glass, green water is made of countless single-celled algae suspended in the water column, each too small to see alone and impossible to wipe away. Under strong light they multiply until the tank looks like pea soup and the back wall disappears. The reassuring news is that these algae are not toxic to fish, and by consuming ammonia and nitrate they can even contribute a little to water quality. The problem they announce is imbalance, and the loss of any view of your fish is reason enough to solve it.
The two ingredients every green bloom needs
Floating algae need light and food together, and removing only one rarely ends the bloom. Light problems are usually obvious once you look: a tank catching direct sun from a window, lights left on from morning to midnight, or a new, stronger light fitted without shortening the day. Food means dissolved nutrients, mainly nitrate and phosphate, which build up from fish waste, overfeeding, decaying plant matter, and in some areas tap water that arrives already carrying phosphate. A bloom after a holiday often has both authors: the feeder overfed while the timer kept a long day.
Why water changes alone do not fix it
It is tempting to change water until the green dilutes, and a large change does clear the view for a day or two. But each refill returns fresh nutrients, the surviving algae cells divide quickly in the light, and the tank greens again on schedule. Keepers sometimes repeat this cycle for weeks while concluding their case is stubborn, when really the two inputs never changed. Water changes earn their place after the light period is cut and feeding is honest, as a way to export the nutrients already dissolved while the algae population starves down.
The fix, in the order that works
Start with light: cut the lighting period to a modest daily window, move or shade the tank if sunlight reaches it, and consider a few days of complete blackout wrapped around the tank for a heavy bloom, since floating algae cannot photosynthesize in the dark while fish and hardy plants tolerate a short fast from light. Test nitrate and look honestly at feeding quantities; most community tanks are fed more than they need. Then change water in sensible partial steps. If the green persists despite corrected light and lean feeding, or fish show distress, clamped fins, or surface gasping alongside it, test ammonia and contact an aquatic vet or good fish store, because something beyond algae is stressing the tank.
Expect the pattern, and record it
Green water loves to return with the seasons, when lower sun angles reach tanks that were shaded all winter. If yours comes back at the same time each year, the cause is architectural rather than a failure of effort. Logging the outbreak dates, nitrate readings, and what you changed, in the Pet Passport on KinsLe, builds exactly the record that reveals that pattern, so next year's bloom gets a lighting fix on day one instead of a month of water changes.