16/02/2026
Last week we visited a pool owner customer, who had their pool plastered in Blue Lagoon back in January. They'd noticed a couple of recent small brown stains in the grout on a longer stretch of wall.
After an onsite look, it was clear the marks were algae that had penetrated into the grout during an early period where chlorine levels had been very low (the owner had been testing and keeping records as we always recommend).
Algae can present brown or green, and because grout is more porous than plaster, it can settle slightly below the surface if water balance isn’t right in those first few weeks. Importantly, the plaster itself was still in excellent condition with no signs of algae establishing on the finish.
We tested removal of the stain progressively (nylon brush, wire brush, then pumice) and were able to remove the affected surface layer of grout to address the problem. Whilst this removed the current symptom, we advised that they carefully monitor sanitation levels (for their scenario, it was chlorine but it's system dependent) and speak immediately with their pool technician about a possible shock treatment if there are any signs of recurrence. You can remove algae from a surface but it often needs to be killed off in the water to prevent recurrence. To assist further, we also redirected a return jet to give more circulation to the wall in question.
In short, in the early stages after plastering, sanitation levels must be kept consistently within the recommended range to prevent this kind of thing. Even a short period of low levels can allow algae to establish in porous areas, particularly in the curing stage following plastering.
Regular brushing, good circulation and working with an experienced pool care professional to manage water chemistry makes all the difference.
The owner was pleased we were able to identify the cause and provide clear guidance going forward. Overall, their pool was looking great and settling exactly as expected. Aside from addressing the low chlorine early on, they'd been very diligent.
Post-install inspection of a newly plastered Rainbow Quartz pool revealed minor grout staining. We explain the cause, treatment and how to prevent recurrence.
