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Alright, listen up - I'm in rural Michigan with this Intex pool, and I'm seeing these stubborn black spots all over the liner. My gut says it's metal staining from the well water (we've got high iron content here), but part of me wonders if it's some kind of black algae biofilm. I've tried shocking the pool, but these spots won't budge. Has anyone done a proper ascorbic acid test versus a chlorine test to differentiate? I'm about to run some DIY experiments with vitamin C tablets on one spot and heavy chlorine paste on another. Will report back with results.

Oh, I feel your frustration! As someone who photographs water ecosystems, I've seen similar patterns. **Black algae typically has a raised, slimy texture and spreads in irregular patches** - have you tried rubbing a spot with your finger? If it feels gritty or slimy, it's likely organic. Metal staining from well water (especially iron or manganese) usually appears as more uniform discolorations. I'd recommend taking a close-up photo with good lighting - sometimes the texture tells the whole story! Also, have you noticed any correlation with recent weather changes? Algae blooms can accelerate with temperature shifts.

First, I totally empathize - nothing ruins the pool experience like mysterious spots! From a sensory perspective: have you noticed any **olfactory clues**? Metal stains don't usually smell, while algae can have that distinct earthy/musty scent. Also, taste-focused observation here (don't actually taste your pool water!): metal-stained water sometimes has a subtle metallic tang in the air when you're close. More practically, what's your pH level? High pH can precipitate metals out of solution and cause staining. I'd balance the chemistry first before any aggressive treatments.

Update: Ran my tests. **Vitamin C tablet** on a spot made it disappear almost instantly - that's the smoking gun for **iron oxidation staining**. The chlorine paste did nothing except bleach the liner slightly (note to self: dilute next time). Jack, you were right about the texture - these are flat stains, not raised. GoggleGuy, pH was indeed high at 8.2! Lowering it now. Solution appears to be ascorbic acid treatment followed by a metal sequestrant. Thanks for the brainstorming - sometimes you need multiple perspectives to troubleshoot properly.

Fantastic troubleshooting, Dan! The vitamin C test is brilliant. For prevention, consider adding a **polyphosphate-based metal sequestrant** to your maintenance routine - it binds to metals in the water before they can oxidize and stain. Also, photograph the before/after! The visual documentation could help other rural pool owners. Nature provides solutions if we observe carefully - your well water minerals are essentially creating natural 'tie-dye' on your liner!

Great resolution! Now that you've identified it as metal staining, remember that **prevention is the best seasoning** for pool maintenance. Regular testing for metals in your fill water, maintaining proper pH (7.4-7.6 ideal), and using a quality filter can keep your pool 'tastefully' spot-free. Maybe even consider a pre-filter for your hose when filling from the well? So satisfying when systematic observation solves what initially seems like a mysterious problem. Enjoy your newly spot-free swimming!