What it looks like
An even, light green film across the entire glass, like fogging. It wipes off easily with a cloth—and reappears in a day or two, just as even. It's this persistence that distinguishes it from spot algae, which is hard but grows slowly.
Plants usually feel fine; the film settles on the glass, not on the leaves.
What causes it
Most often, it appears in aquariums that are three to six months old, following brown algae. This is a transitional state: the biology is already working, but equilibrium has not yet been established.
What to do
Do not touch for three weeks. This is the only case where inaction is an effective strategy, and it works like this: this algae has a motile stage. As long as you wipe off the film every two days, you repeatedly release fresh spores into the water, and the cycle repeats endlessly. If you let the film mature, it detaches from the glass on its own, settles, and does not return.
For three weeks, you'll have to look through green glass. Many people can't stand it and start wiping in the second week—and everything starts over.
While you wait:
- The front glass can be wiped once before guests arrive, but leave the others untouched.
- Water changes should be done as usual; this does not spoil the water.
- Lighting and fertilizers should not be changed; they are not the cause.
If the film has not disappeared after a month, then the problem is indeed an excess of light—shorten the photoperiod to 7–8 hours and wait another two weeks.