What it looks like
Tiny, dark green, pinprick-sized dots, most often on the glass – in the most illuminated part of it – and on old, fully grown leaves. The dots are hard: they don't come off with a fingernail, a cloth won't remove them; a blade scraper is needed.
Hardness is the main characteristic. Brown algae wipes off with a finger, green dust algae with a cloth; these dots must be scraped off.
Why it appears
The main reason is phosphate deficiency under strong light. This sounds unexpected: usually, algae are fought by cutting back on nutrients. Here, it's the opposite – spot algae proliferates precisely where there's almost no phosphate left in the water, but plenty of light.
A second common reason is too long or too bright a photoperiod with slow-growing plants. Light that plants cannot use goes to the algae.
What to do
- Check phosphate. If it's close to zero, raise it with fertilizer to 0.5–1 mg/L and maintain it at that level. Often, this alone is enough, and new spots stop appearing within a couple of weeks.
- Shorten the photoperiod to 7–8 hours if it was longer. Adjust brightness second: duration first.
- Scrape off what has grown. It won't go away on its own. Clean glass with a blade; old leaves with dense growth are easier to remove entirely – they are at the end of their life anyway.
The order is important: if you scrape first and don't address the cause, the spots will return to the same place within a week.
What not to do
Do not cut back on nutrients "just in case." With this type of algae, phosphate deficiency is the cause, not the consequence, and a starvation diet makes it worse. This is a case where the general rule "less fertilizer – less algae" does not apply.