Why brushing matters at all
Algae in a pool is a biofilm anchored to the surface, and its outer layer shields the cells beneath from chlorine. That is why a green patch survives a shock and returns in the same place. Mechanical abrasion breaks the film open so the sanitizer can reach the organism — which is a thing chemistry alone cannot do.
A robot with an active scrubbing brush does this across the floor and, on a climbing machine, the walls. What it does not do is the awkward geometry, and the awkward geometry is also where circulation is poorest, which is exactly where algae establishes.
The seven places no robot reaches
- Steps and benches. Vertical faces and horizontal treads in quick succession. Almost no robot handles them; platform-mode machines (Beatbot Sora 30, Sora 70) do more than most.
- Behind and around ladders. A shaded enclosed pocket with no flow. One of the two most reliable algae sites in any pool.
- Light niches. Recessed, warm, shaded. The other reliable algae site.
- The skimmer throat and faceplate. Water passes through constantly and film builds on the plastic.
- Tight corners, especially in freeform pools. A robot’s turning radius leaves a wedge.
- Under return fittings. Oddly, high-flow areas build film at their edges.
- The last inch under the coping. A brush cannot reach the angle. The waterline guide covers this.
How often, with a robot
| Area | With a floor-only robot | With a climbing robot |
|---|
| Floor | Monthly spot-check | Not needed |
| Walls | Weekly | Monthly spot-check |
| Waterline | Weekly | Fortnightly, or not if it has a waterline pass |
| Steps and benches | Weekly | Weekly |
| Behind ladders, light niches | Weekly | Weekly |
| Corners and skimmer throat | Fortnightly | Fortnightly |
So: five or ten minutes a week with a pole brush on the specific spots, rather than twenty minutes on the whole pool. That is what a robot buys you here.
Surface changes the answer
- Plaster and gunite: porous and alkaline, so algae gets a foothold easily. Brush more. Black algae roots into plaster and needs a stiff brush plus targeted treatment — no robot handles it. See the pebble and gunite page.
- Pebble and exposed aggregate: texture holds film in the valleys between stones. Brush more, and expect never to get it perfectly clean.
- Fiberglass gelcoat: non-porous, so film sits on the surface and lifts easily. Brush less, and use nothing abrasive — gelcoat scratches permanently.
- Vinyl liner: brush gently with a soft brush. Never a stainless brush, which will mark the liner — see the liner page.
- Tile: easiest surface; a brush pass plus balanced chemistry keeps it clean.
Brush choice matters as much as frequency
Nylon brushes for vinyl and gelcoat. Nylon-and-stainless combination brushes for plaster and gunite only. Never stainless on vinyl or fiberglass. The CDC’s healthy-swimming guidance is the reference for the chemistry side of the equation; the brush handles the mechanical side.
Brush before you shock, not after
The order matters. Brush the affected surfaces first to break the biofilm, then shock so the chlorine reaches the exposed cells, then run the robot with a fine filter to remove the dead algae as the water clears. The algae roundup sets out the full sequence.
