What it looks like
The water becomes cloudy and green — from a slight haze to a state where the back wall is invisible. At the same time, glass and leaves remain clean: the algae does not live on surfaces but in the water column, floating freely.
This is how you distinguish it: if the glass itself is green — it's spot or dusty algae, if the water is green with clean glass — it's an algae bloom.
Why it appears
The trigger is almost always an ammonia spike combined with strong light: replanting with substrate disturbance, cleaning the substrate after a long break, fish death, filter restart. Separately — direct sunlight on the aquarium.
What to do
Water changes do not help — this is the first thing you need to know. The algae divides faster than you can dilute it, and after a water change, fresh nutrients only accelerate it. People change half the volume daily for weeks and are surprised that it gets worse.
Two methods work:
Blackout for 4–5 days. The aquarium is completely covered from light, the filter is left on, feeding is sparse or not at all. If there are fry in the aquarium, they are fed briefly under a flashlight and immediately covered again. Floating algae dies without light in a few days, plants tolerate this normally. Afterward — a water change and filter cleaning, which will collect the dead algae.
UV sterilizer. It is placed at the filter output and deals with the bloom in two to three days, reliably and without darkness. It costs money and is needed once every few years — many rent it or borrow it from acquaintances.
After the water has cleared, find the cause. If it was the sun — shade the wall. If it was substrate disturbance — in the future, do it in parts and follow with a water change.
What not to do
Do not change water every day. This is the most common and most useless course of action for algae blooms: it takes weeks and makes things worse.