Rule out stains and metals first. Dark or yellow marks are not automatically algae.
- ✕Do not start by guessing — the wrong diagnosis wastes chlorine
- ✕Do not use a harsh tool that damages vinyl or fiberglass just because the stain is stubborn
- ✕Do not stop treatment before verifying the problem is actually gone
FC / CC / CYA / pH
Black and Mustard Biofilm
Treat black algae, mustard/yellow algae, slime, and stubborn biofilm as a diagnostic and mechanical-cleaning problem before you keep raising chlorine.
Rule out stains and metals first
A dark or yellow mark is not automatically algae.
Find the hidden niches
The problem usually survives where the brush and sanitizer do not reach often enough.
Break the layer mechanically
Chlorine alone is usually not enough when the growth is protected by a surface layer.
- Do not use a harsh tool that will damage vinyl or fiberglass just because the stain is stubborn.
Use sustained remediation, not a one-time spike
If it is truly algae or biofilm, the treatment needs staying power.
Verify the problem is actually gone
A partial cleanup is not a finish line if the same spots come back.
Resources (5)
Stains, metals, and discoloration
Rule out metal staining, scale, and finish issues before you commit to an algae cleanup.
Pool water testing and accuracy
Use the testing-accuracy guide when you need a trustworthy chlorine, pH, or CYA baseline before remediation.
Filtration and circulation
Use the circulation guide when dead spots and turnover are part of the hidden-growth problem.
SLAM
Use the remediation workflow when the diagnosis really is algae and the pool needs sustained treatment.
Clear cloudy water
Use the lighter clarity workflow when the symptom is haze rather than a stubborn growth pattern.
Educational guidance only. Verify labels, manuals, local code, and site conditions before acting. Stop for electrical, gas, structural, drain, drowning, injury, emergency, or chemical-mixing risk.