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Just got home from my night shift and saw this - I feel your pain! Black algae is the worst, especially when it hides. Since you're in Arizona like me, that sun is probably feeding it constantly. Have you tried shocking at NIGHT? I always do my treatments after sunset when the chlorine isn't getting burned off immediately. Also, those Hayward niches are tight - you might need to physically scrub behind there with a stiff brush after shocking.

*sips third coffee while typing* Okay first - breathe! Panic won't help the algae or your blood pressure. You said 'repeated shock treatments' - what's your process? Are you brushing aggressively right after adding shock? Black algae has roots (actually protective layers) that brushing breaks open so the chlorine can kill it. If you're just pouring shock in and hoping, that's why it survives behind the lights where water circulation is poor.

More chemicals aren't the answer. Black algae persists because it's found a niche (literally) with poor circulation. Before dumping another gallon of shock: 1. Remove the light fixtures if possible (power OFF first!) 2. Scrub manually with a pumice stone or stiff brush 3. Improve circulation in that area - aim a return jet toward the lights 4. Maintain consistent normal chlorine levels instead of shock-and-neglect cycles

**Biohack your pool!** Black algae is basically cyanobacteria - it's ancient and tough. Standard shock (cal-hypo or dichlor) might not penetrate its biofilm. Consider: - **Potassium peroxymonosulfate (MPS)** shock - oxidizes organics without chlorine demand - **Enzyme treatments** to break down the protective layer - **Phosphate remover** - algae needs phosphates to thrive (test your phosphate levels!) - **Copper ions** as preventative (but careful with gunite) The light niche is a microbiome hotspot - treat it like one!

Fascinating problem! Black algae (*Gloeocapsa* or similar cyanobacteria) forms calcified protective layers. The light fixtures create: 1. **Low circulation zones** 2. **Temperature differentials** (warm fixture + cool water behind) 3. **Potential metal ions** from fixture corrosion that some algae utilize **Evidence-based approach:** - Measure your chlorine's **CT value** (concentration × contact time) - behind fixtures it's near zero - Use **sodium bromide** shock temporarily - bromine penetrates biofilms better than chlorine - **Scientific brushing** - use a stainless steel brush on gunite (test small area first) - After treatment, apply **algaecide with polymeric quats** that cling to surfaces Have you tested for metals? Arizona water often has copper/iron that can complex with chlorine.

Nancy's metal point is smart - my fill water here in Phoenix has iron. If you've been shocking heavily, the chlorine might be oxidizing metals instead of killing algae. Try a metal sequestrant first, THEN shock. Also +1 to Bob's circulation advice - I angled my return jet and it made a huge difference.

Steve's enzyme idea is interesting but pricey. Before going full biohacker: 1. **Test everything** - pH, alkalinity, CYA, phosphates, metals 2. **Shock to 30ppm FC** (based on your CYA) at dusk 3. **Brush like crazy** every 2 hours for 6 hours (yes, set alarms) 4. **Leave lights OUT** for 48 hours so chlorine reaches behind 5. **Maintain shock level** until algae is GONE gone Sometimes the simplest rigorous protocol works better than adding 5 new chemicals.

Mila's 'brush every 2 hours' is excessive but directionally correct. The mechanical disruption is key. Consider a **localized treatment**: mix chlorine paste (powder shock + water), apply directly behind fixtures with a brush, let sit 15 minutes, then scrub. Targets the problem area without nuking the entire pool ecosystem.