Algae fix guide
Green Spot Algae
Also called GSA, green dot algae
Hard little green dots on the glass and on old, slow-growing leaves.
By Mike Elmira
TL;DR, Green Spot Algae
Green spot algae shows up as small, firm green dots stuck to the glass and to slow leaves like anubias. It is one of the tidier problems to have, and it usually points at low phosphate rather than too much of anything. Bump your phosphate dosing up a touch, keep the glass scraped, and let nerite snails rasp what is left. On plant leaves it is mostly cosmetic, so trim the worst leaves and move on.
How to spot it
Small, round, deep green dots, hard enough that they need a scrape or a blade to shift. They cling to the glass and to the oldest leaves on slow growers.
What causes it
- Low phosphate (PO4) in the water column, the most common trigger
- Strong light landing on slow-growing leaves and bare glass
- Old leaves that have been sitting under light for weeks
How to fix it
- Raise phosphate dosing slightly and keep it steady, this alone clears most cases over a few weeks.
- Scrape the glass with a blade or a firm pad; a credit card edge works on thin glass.
- Trim the worst-affected old leaves rather than fighting to clean them.
- Add a couple of nerite snails, they graze hard green spot off glass better than anything.
Keep it from coming back
Keep phosphate topped up and don't run light brighter or longer than the plants can use. A tank that is dosed steadily rarely gets much of it.
What eats it
A clean-up crew won’t fix the cause, but these species graze it and keep the last of it down:
Frequently asked questions
Direct answers to the questions search engines and AI assistants surface most often about this species.
Is green spot algae bad for my tank?
No, it is harmless to fish, shrimp and plants. It is mostly a look problem on the glass and old leaves, and it is one of the easier types to keep in check.
Why do I keep getting green spot algae on the glass?
Almost always low phosphate. Raise your phosphate dosing a little and hold it steady, and the dots slow right down. Scraping plus a nerite snail handles the rest.