Why algae always come back to the same spot in the pool (and what that spot is trying to tell you)

You remove them, and they come back. Always in the same place: on the waterfall stones, on the shallow area facing south, near the filter outlet. It's neither chance nor a curse. When algae always choose the same area, it's because they find exactly what they lack elsewhere: more light, more current, or more nutrients. In other words, algae are almost never the real problem: they point you toward the cause.

Algae are almost never the real problem

Algae only grow when they have light, nutrients, and a suitable surface. If they keep coming back to the same place despite your efforts, it means these conditions are permanently present there. Pulling out algae without changing these conditions is like mowing a lawn: it grows back. Conversely, changing just one condition in the affected area is often enough to shift the advantage toward plants or cleanliness. This is a much more effective, and far less costly, approach than treating the entire pond.

Reading algae-prone zones

The waterfall case

This is the most common and most counterintuitive area. You might think that moving water prevents algae, but filamentous algae actually thrive in flowing, well-oxygenated water, like in mountain streams. On a waterfall, the current constantly brings fresh nutrients to algae attached to the rocks in full light: it is an all-you-can-eat buffet. Regular brushing in spring, plants along the stream, and partial shade greatly limit their growth. Above all, reducing nutrients throughout the pond water decreases regrowth on the waterfall.

The feeding area, an underestimated hotspot

Many owners always feed in the same spot, often near the terrace, where the water is shallow and clearly visible. This is perfect for observing the fish, but it is also where leftovers fall, droppings accumulate, and silt builds up fastest. The result: a mat of algae that constantly grows back at the foot of the terrace. Feed only as much as can be eaten within a few minutes, near a skimmer or intake if possible, and regularly vacuum the bottom in that area.

The targeted, zone-by-zone method

1. Map the areas where the algae return, in different seasons.
2. Identify the dominant factor: light, current, nutrients, or several at once.
3. Remove the algae mechanically and take them out of the pond.
4. Install competitors: submerged or marginal plants suited to the depth.
5. Cut off the source: shade, relocated feeding spot, poor substrate, diverted runoff.
6. Reduce nutrients throughout the pond so the area loses its advantage.

Mistakes that maintain these areas

• Leaving removed algae on the bank, where rain carries their nutrients back into the water.
• Planting in garden soil or with soluble fertilizer.
• Treating the entire pond when the problem is localized.
• Cleaning the waterfall with bleach or a pressure washer, which damages the surfaces and encourages recolonization.

Read also

To understand filamentous algae: Why do I have filamentous algae in my pond?.

READ THE ARTICLE

Frequently asked questions

— Why are there always algae on my waterfall?

Because running water and light continuously bring nutrients and oxygen to the attached algae. Brush regularly and reduce the pond's nutrients.

— Do algae in the same place indicate a fertilizer leak?

It is possible, especially near a planting basket or a lawn-side edge. Check the substrate and runoff.

— Should the entire pond be treated?

Rarely. Targeted action in the area, combined with overall nutrient reduction, is more effective.

— How long does it take for the area to stay clean?

Often a few weeks after eliminating the cause, while the plants take over.

In summary

• Algae return where light, current, and nutrients accumulate.
• Waterfall, sunny area, dining corner, and filter return are typical areas.
• Each area indicates a specific cause to correct.
• Targeted action and overall nutrient reduction: the combination that works.

Last updated: September 28, 2026.