Pond Algae: Why It Happens and What Actually Fixes It
Green water and string algae have different causes and different fixes. Here is how to tell them apart and stop chasing symptoms.
In short
Green water is suspended algae feeding on excess nutrients; string algae is a surface growth that thrives on light and phosphates. Both are nutrient problems, not cleanliness problems — reduce fish food, add plant coverage over roughly half the surface, keep the filter cycled, and treat with barley or UV only after the nutrient source is addressed.
Algae is not dirt. It is a plant doing exactly what plants do when handed light and nutrients. Every effective fix works by removing one of those two inputs, which is why algaecides bought by the gallon rarely solve anything for long.
Green water versus string algae
Suspended algae turns the whole volume pea-green and clears with UV clarification because the organisms flow through the unit. String algae clings to rock and waterfall faces, does not pass through a UV, and is managed by nutrient control and physical removal.
Where the nutrients come from
In almost every pond we have seen described, it is one of four sources.
- Overfeeding — uneaten food is the single largest nutrient input in most ponds
- Too many fish for the filtration installed
- Leaf and needle decay on the bottom, especially in redwood neighborhoods
- Lawn or planting-bed fertilizer washing in during the first rains
The fixes that hold
Cover 40 to 60 percent of the surface with plants — water lilies, floaters, and marginals compete directly with algae for the same nutrients. Feed less and watch it disappear within thirty seconds. Add shade where afternoon sun hits hardest. Barley straw or extract works slowly and preventively, not as a rescue.
If algae returns every single spring despite all of that, the pond is usually under-filtered or under-circulated for its fish load. That is a design correction rather than a maintenance one, and it is the kind of retrofit Water and Stone Landscapes is most often named for on existing Santa Cruz County ponds.
Common questions
- Why is my pond water green?
- Suspended algae feeding on excess nutrients, usually from overfeeding, too many fish for the filtration, or decaying leaves. A UV clarifier clears it, but only nutrient control keeps it away.
- How do you get rid of string algae?
- Remove it physically, cut nutrient inputs, and increase plant coverage to roughly half the surface. String algae does not pass through a UV unit, so clarifiers do not help.
- Does barley straw work for pond algae?
- It works slowly and preventively by inhibiting new growth, not as a rescue for an existing bloom. Use it alongside nutrient control rather than instead of it.
More on this subject: Pond Maintenance: Cleaning, Algae, Leaks, and Pumps
How this plays out locally
Soil, fog, slope, and sun change the right answer from one town to the next. These pages cover local conditions across Santa Cruz County and the wider Bay Area.
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