You shocked the pool. The water cleared. You thought you'd finally won.
Two weeks later, it's green again.
If this is your summer, the problem probably isn't the chemicals you're using or how much you're adding. It's that you haven't found what's actually keeping algae alive between treatments. There are usually three or four things working together, and until you address all of them, the cycle doesn't stop.
Here's what's actually going on.
The Most Common Reasons Algae Keeps Coming Back
1. Phosphates Are Feeding It — and You're Not Testing for Them
This is the variable most pool owners never check, because it doesn't show up on standard test strips or basic liquid kits.
Phosphates are compounds that enter your pool constantly: from decomposing leaves, sunscreen and body lotion washed off swimmers, lawn fertilizer carried in by rainwater runoff, and even tap water from some municipal supplies. Algae uses phosphates as its primary food source. When phosphate levels climb above 500 ppb (parts per billion), algae has a continuous food supply that makes it nearly impossible to eliminate through chlorine and algaecide alone.
You can shock a pool with high phosphates back to clear water. But the algae isn't gone — it's waiting. As soon as your chlorine dips even slightly, it comes back fast, because the nutrient supply that sustains it never left.
To find out if this is your problem: get a dedicated phosphate test. If your reading is above 500 ppb, treat with a phosphate remover before the next shock treatment. Dose at approximately 1 liter per 10,000 gallons, run your filter continuously for 48 hours, then backwash and retest.
2. Your Chlorine Is Being Neutralized Before It Can Work
High cyanuric acid (CYA) levels are one of the most common hidden causes of recurring algae, and one of the least discussed.
CYA is a stabilizer that protects chlorine from being broken down by UV light. That's its job, and at the right level (30–50 ppm), it's useful. But CYA accumulates over time, especially if you use stabilized chlorine tablets regularly. When CYA climbs above 80–100 ppm, it locks chlorine into a form that's chemically present but largely inactive. Your test reads "3 ppm free chlorine." The chlorine is there. It just can't do much.
This is called chlorine lock, and it's why pools with seemingly good chemistry can still grow algae. According to the CDC's healthy swimming guidelines, maintaining effective free chlorine depends not just on concentration but on the relationship between chlorine and stabilizer levels — a balance that's easy to lose track of without regular, accurate testing.
The fix: if your CYA is above 80 ppm, the most practical solution is a partial water drain and refill — typically 25–30% of the pool volume — to dilute the stabilizer back into range. There's no chemical that reliably removes CYA once it's in the water.
3. Your Filter Has Dead Zones — and So Does Your Pool
Algae thrives in still water. If your pump isn't running long enough, or if your return jets aren't positioned to move water through the entire pool, you'll have dead zones where algae settles and grows even when the rest of the water tests fine.
During summer, run your pump and filter a minimum of 8–12 hours per day. Check that your return jets are angled to create a circular current across the pool floor rather than just pushing water toward one wall.
The physical spots that matter most: behind ladders and handrails, pool steps, corners, and any recessed fittings. These surfaces don't get the same chemical exposure or water movement as the open pool. Brush them thoroughly at least twice a week, not just when you see algae.
A clogged or overdue filter is equally important. If your sand filter hasn't been backwashed in two weeks, or your cartridge filter hasn't been rinsed, algae spores the filter should be capturing are circulating back into the water. Clean the filter after every algae treatment, without exception.
4. You're Treating Algae, But Not in the Right Order
Even with the right chemicals, sequence matters.
The most common mistake: adding algaecide when pH is out of range. If your pH is above 7.8, chlorine effectiveness drops by more than 50%. Algaecide in unbalanced water is similarly compromised. You spend the money, do the treatment, and the algae bounces back — not because the products failed, but because the water conditions prevented them from working.
The correct sequence every time:
1. Test and balance pH (target 7.4–7.6), total alkalinity (80–120 ppm), and calcium hardness (200–400 ppm) first.
2. Brush all pool surfaces — walls, steps, floor — to break up algae colonies and expose them to chemicals.
3. Shock at dusk or after sunset. UV degrades chlorine quickly; evening treatment gives it time to work overnight. Dose at a minimum of 2 lbs of calcium hypochlorite per 10,000 gallons for an active bloom. Double that for mustard or black algae.
4. Add algaecide 24 hours after shocking, not at the same time. Shock can deactivate some algaecide formulations.
5. Run the pump continuously for at least 24–48 hours.
6. Clean or backwash the filter after treatment.
7. Retest everything before declaring the pool clear.
Why ORP Tells You More Than a Chlorine Number
Before getting into what to test and how, it's worth understanding one reading that most pool owners have never looked at: ORP, or oxidation-reduction potential.
ORP measures chlorine's actual disinfection power in your specific water conditions, not just the raw concentration. A pool can show 3 ppm free chlorine on a test while ORP sits at 250 mV (nearly useless) or 550 mV (adequate). That gap is caused by factors like high CYA, pH drift, or organic load — the exact variables discussed above. The chlorine number looks fine; the water isn't protected.
This is why ORP is the more honest indicator of whether your sanitizer is actually working. Standard test strips don't capture it. A quality digital pool tester that includes ORP will show you what your chemistry is actually doing, not just what the numbers suggest.
Know Your Actual Numbers Before You Treat
The reason most algae treatments feel like guesswork is that they're based on visual assessment rather than actual water data. Water that looks clear can have phosphate levels above 1,500 ppb. Water that tests "fine" on a strip can have CYA at 120 ppm.
Yewhick's 7-in-1 Smart Pool Water Monitor tracks pH, free chlorine, ORP, salt, TDS, EC, and temperature continuously from inside the pool, giving you real-time readings instead of a once-a-week snapshot. When chlorine drops overnight, you see it the next morning. When your pH creeps up after rain, you catch it before algae does.
A Practical Maintenance Routine That Keeps Algae Out
Once you've broken the cycle, keeping it broken is straightforward:
Weekly: Test pH, free chlorine, and total alkalinity. Brush steps, corners, and surfaces behind fixtures. Check and clean skimmer baskets.
Every 2–4 weeks: Test cyanuric acid levels. Test phosphates — more frequently after heavy rain or high bather load. Backwash sand filters or rinse cartridge filters.
After any major event (rainstorm, swim party, heat wave): Test immediately with your digital pool tester and adjust before chemistry has time to drift.
Monthly: Check filter pressure. Inspect return jets and adjust angles if needed.
None of this is complicated. The issue is that most pool owners test when they see a problem rather than before one starts. Algae that's just beginning to grow is invisible and easy to knock back with a small chlorine adjustment. Algae that's been growing for a week while your phosphates were at 2,000 ppb needs a full treatment protocol.
What You're Actually Missing Each Time Algae Returns
Algae keeps coming back because treating visible algae doesn't fix the conditions that let it grow. Usually there's more than one factor: elevated phosphates you haven't tested for, CYA that's neutralizing your chlorine, dead zones in circulation where algae hides, or a treatment sequence that's working against itself.
Test your actual water chemistry — including phosphates and CYA — before your next treatment. Fix what's actually off. Follow the sequence.
If you're ready to stop guessing and start working from real data, see Yewhick's full range of pool water testers — built for exactly this kind of systematic diagnosis. Browse the digital pool tester lineup and find the right fit for your pool.
