Black Algae: Can You Accidentally Infect Another Pool?

Black algae in a swimming pool is one of those problems that feels personal because it fights back. The spots look like tiny black pinheads on plaster, but the real issue is what’s underneath. As the podcast explains, “black algae” is often cyanobacteria, not true algae, and it can anchor itself deep into porous surfaces. That helps explain why it returns when chlorine drops, and why simple “shock it and forget it” pool maintenance fails. If you’re a pool owner or a pool service professional, the goal is twofold: kill what you see and prevent black algae from spreading to the next pool through cross contamination.

A key takeaway is how black algae spores can enter water in the first place. The idea that it comes from nature, including oceans and lakes, is plausible because spores can cling to wet fabric and damp gear for surprisingly long periods. A wet bathing suit, towel, or T-shirt can carry spores from a beach day right into a backyard pool. The same is true for pool cleaning tools that stay damp between stops, like leaf rakes, nets, and brushes. If you want black algae prevention, separate “ocean gear” from “pool gear,” rinse and dry items thoroughly, and stay vigilant when you see the earliest signs of dark spotting on plaster or tile.

Treatment needs both mechanical removal and strong sanitizer strategy. The episode stresses that brushing is mandatory because the organism forms a protective coating that blocks chemicals. A stainless steel algae brush is often needed to break the heads off so chlorine can penetrate. From there, superchlorination becomes effective, with the host recommending very high free chlorine targets for a short period, plus strict safety rules so nobody swims during high chlorine. Topical options that sit directly on the spots can help too, including granular trichlor products applied carefully, or broken cal-hypo tablets as a slower-dissolving alternative that is less likely to damage plaster when used correctly.

For long-term control, the conversation highlights two big risk factors: low sanitizer levels and contaminated equipment. A pool running at 1 ppm free chlorine or less is far more likely to “catch” black algae if spores are introduced. That is why route discipline matters for pool service: dedicate a net, leaf rake, and especially a stainless steel brush to any account with black algae, and avoid reusing bags or cloth-like tools that hold moisture. After cleanup, prevention is about consistency: maintain proper chlorine so it never “zeros out,” consider a carefully managed copper algaecide or a chelated copper system like PoolRx to reduce regrowth risk, and treat any new spots early before they spread into a bigger, more expensive black algae removal job.