The short answer
Green spot algae (GSA) appears as small, hard green discs on the glass and on slow-growing leaves like anubias. Its two main drivers are high light and low phosphate (PO4). Balance those — usually by trimming light and adding a little phosphate — and GSA slows dramatically.
Why it appears
GSA thrives when lighting is strong or runs long but a key nutrient, phosphate, is in short supply. That imbalance stresses plants and gives this particular algae an edge. It shows up first on the oldest, slowest surfaces — the glass and tough leaves like anubias and java fern — because they don’t grow fast enough to shed it.
Common triggers: an intense light over a lightly-dosed tank, or a long photoperiod without matching nutrients.
How to fix it
- Dose a little phosphate: raising PO4 into a modest range often visibly reduces GSA. A complete fertilizer usually includes it — see the fertilizer hub and our recommended plant fertilizer.
- Reduce light: shorten the photoperiod to 6–8 hours or lower intensity. Browse the lighting hub or a gentler budget light.
- Scrape the glass: GSA is hard, so use a blade scraper or magnet — see how to get algae off aquarium glass.
Keep it in check
Nerite snails graze GSA better than most, though the hardest discs resist even them. The durable fix is a balanced tank: light matched to nutrients, with healthy plants using up what’s available. For the wider plan, see how to get rid of aquarium algae.