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Hey all! As a digital nomad who's managed pools in three different climates, I've seen this exact issue. You're passing visual clarity tests because the algae is microscopic or in the biofilm stage - it's there, just not visible yet. The OCLT (Overnight Chlorine Loss Test) is failing because your chlorine is getting consumed by something overnight. Have you tested phosphates? Oregon's tap water can be high in phosphates, which algae love. Also, check your CYA (cyanuric acid) levels - if they're too high, your chlorine becomes less effective even at proper FC levels.

As a granola parent who avoids harsh chemicals, I'd suggest looking at your pool's ecosystem balance. Clear water doesn't mean healthy water! Have you considered adding beneficial bacteria products? They outcompete algae for nutrients. Also, what's your filtration schedule? With Oregon's cooler temps, you might need longer run times. And please tell me you're not using stabilized chlorine tablets constantly - they build up CYA which Nancy mentioned. Try a phosphate remover (there are plant-based ones) and increase circulation.

Biohacker perspective here: This is a classic case of incomplete sanitation. You're treating symptoms, not the system. First, do a proper OCLT: test FC at dusk, then at dawn before sunlight. If you lose >1ppm FC overnight, you have organics consuming chlorine. Shock to SLAM level (Shock Level And Maintain) based on your CYA using the [FC/CYA chart](https://www.troublefreepool.com/chlorine-cya-chart). Most importantly, scrub EVERY surface - ladder, skimmer throat, behind lights. Algae hides in biofilms that resist normal chlorine levels.

Eco-warrior chiming in! Before dumping more chemicals, consider environmental factors. Oregon's spring brings pollen and organic debris that feed algae. Are you using a solar cover? It can create warm microclimates perfect for algae. Also, test your fill water - municipal water often contains phosphates and nitrates. Try adding enzymes to break down organics before they become algae food. And remember: sunlight + nutrients = algae. Reduce either one. Shade your pool partially or use a UV system (though I prefer natural solutions).

Following up on Steve23's point about environmental factors - yes! The Pacific Northwest has unique conditions. Morning dew adds freshwater daily, diluting your chemistry. Do you test and adjust after rain/dew? Also, what's your bather load? Even one swimmer introduces organics. Back to testing: what are your exact numbers? FC, CC, CYA, pH, TA, CH, phosphates? Without those, we're guessing. And have you deep-cleaned your filter recently? Intex filters need frequent attention.

I'll add that 'clear water' can be deceptive with certain algae types. Have you done the bucket test? Fill a bucket with pool water, add your normal chlorine dose, and leave it in the sun. If algae grows there but not in your properly maintained pool, it's environmental. If it grows in both, it's your maintenance. Also, consider switching to a mineral system or adding borates (50ppm) as an algaestat - they're less harsh than constant shocking. And please, clean those pool toys! They're algae taxis.

**Action plan based on all inputs:** 1. Test everything: FC, CC, CYA, pH, TA, CH, phosphates, nitrates 2. Perform OCLT properly tonight 3. If OCLT fails, SLAM immediately using liquid chlorine (not tabs) 4. Clean all surfaces including hidden areas 5. Clean/backwash filter 6. Consider phosphate remover if levels >500ppb 7. Adjust pump runtime to 24/7 during treatment 8. Retest OCLT until it passes The key is consistency - algae returns because treatment stops too soon. Continue SLAM until: CC≤0.5, OCLT passes, AND water is clear. Not just one or two conditions.