Algae
Hard green dots are growing on the glass
Green spot algae are small, hard, firmly attached dots that will not wipe off with a cloth. A few on the brightest pane of an otherwise healthy tank are normal and not worth fighting. A dense coating that returns within days points at a long photoperiod, and often at a planted tank running short of nutrients rather than swimming in them.
Watch it
Test these first
Before anything goes into the tank and before anything comes out of it, get these numbers. They cost fifteen minutes and they decide everything that follows.
- Nitrate — the nitrate ion — note that drinking water standards are written as nitrate-nitrogen, which is a different number
- pH and KH — acidity of the water, plus the carbonate buffer that resists changing it
What is usually behind it
| Cause | How to confirm | Time to resolve |
|---|---|---|
| Photoperiod longer than the tank can use | The light runs more than about eight hours, or the tank gets afternoon sun as well. The spots are heaviest on the pane that receives the most light and sparse on the shaded side. | Three to four weeks before regrowth slows noticeably |
| A planted tank short of nutrients rather than overloaded | The tank has real plant mass, nitrate reads low, and older leaves show pinholes or yellowing while green spot spreads across the glass and the slowest-growing leaves. | Four to six weeks for plant health to visibly recover |
| Glass that is only cleaned when it looks bad | The spots are large and layered rather than fine. Once established they harden and become considerably more work. | Immediate for the glass; the habit prevents recurrence |
| Nutrient surplus in an unplanted tank | There are no plants, nitrate sits above 40 ppm, and green growth appears on the glass, the decor and the equipment alike. | Three to five weeks of consistent maintenance |
Photoperiod longer than the tank can use
How to confirmThe light runs more than about eight hours, or the tank gets afternoon sun as well. The spots are heaviest on the pane that receives the most light and sparse on the shaded side.
What to doPut the light on a timer at six or seven hours in one continuous block. Scrape the existing spots with a blade suited to your tank material — a razor on glass, a plastic blade on acrylic, which scratches.
Time to resolve: Three to four weeks before regrowth slows noticeably
A planted tank short of nutrients rather than overloaded
How to confirmThe tank has real plant mass, nitrate reads low, and older leaves show pinholes or yellowing while green spot spreads across the glass and the slowest-growing leaves.
What to doThis is the counterintuitive case: a heavily planted tank starving its plants gives algae the advantage. Feed the plants on a schedule the fertilizer label sets, and keep water changes regular so nothing else accumulates while you do it.
Time to resolve: Four to six weeks for plant health to visibly recover
Glass that is only cleaned when it looks bad
How to confirmThe spots are large and layered rather than fine. Once established they harden and become considerably more work.
What to doMake a pass with a blade or scraper part of every water change, before the growth thickens. Ten seconds weekly replaces twenty minutes monthly.
Time to resolve: Immediate for the glass; the habit prevents recurrence
Nutrient surplus in an unplanted tank
How to confirmThere are no plants, nitrate sits above 40 ppm, and green growth appears on the glass, the decor and the equipment alike.
What to doBring nitrate down with a consistent water change schedule and substrate vacuuming, and shorten the light. Without plants competing, light control does most of the work here.
Time to resolve: Three to five weeks of consistent maintenance
What not to do
These are the moves that make this particular problem worse, and they are all common. Most of them come from wanting to do something substantial when the right answer is something small.
- Do not use a razor blade on an acrylic tank. It will scratch permanently and no amount of polishing fully removes it.
- Do not lengthen the light because the plants look poor. Longer days favor the algae far more than the plants.
- Do not scrape the glass and then leave the fragments to settle. Scrape first, then siphon during the water change.
- Do not treat a handful of dots on the brightest pane as a problem. Every lit tank grows some.
Keeping it from coming back
Green spot is a light-duration problem before it is anything else, so a plug-in timer set to six or seven hours does more than any product on a shelf. Add a scrape to the start of every water change while the growth is still soft, and keep the tank out of direct sun. In a planted tank, watch the plants rather than the glass — leaves in good condition are the best evidence that the balance is where you want it.
Reference for this page
Equipment involved
- Test Kitsknowing the number instead of guessing it
- Filtersturning water over and housing the bacteria
- Filter Media & Cartridgeswhat actually does the filtering, and when it gets replaced
Related problems
Watch it
Act this week
Act this week
Published sources
- US EPA — Nutrient Pollution
- NOAA National Ocean Service — Harmful Algal Blooms
- USGS Water Science School — pH and Water
Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.