Algae treatment costs 10 times more than algae prevention. A pool that turns green requires triple-shocking, extended filtration, multiple return visits, and a customer who's been unable to use the pool for days. Prevention is the better business outcome and the better service outcome. This is the prevention framework that works.
Algae is always present in pool water as spores. The question isn't whether algae is in the water — it's whether conditions allow it to reproduce faster than the sanitizer can kill it. Three conditions enable a bloom:
Summer combines all three: heat is at its peak, bather load is highest (introducing more nutrients and organic contamination), and chlorine burns off faster. Prevention means controlling what you can control — primarily the chlorine-to-CYA ratio.
The active sanitizing agent in pool water is hypochlorous acid (HOCl). CYA binds chlorine and moderates its reactivity — protecting it from UV degradation but also reducing the immediate availability of free HOCl. The higher the CYA, the more total FC you need to maintain adequate active sanitizer.
Minimum FC by CYA level:
| CYA Level (ppm) | Minimum FC (ppm) | Target FC (ppm) |
|---|---|---|
| 20 | 1.5 | 2–4 |
| 30 | 2.3 | 3–5 |
| 40 | 3.0 | 4–6 |
| 50 | 3.8 | 5–7 |
| 60 | 4.5 | 6–8 |
| 80 | 6.0 | 7–10 |
The minimum column is the floor — below this, algae can establish even in the absence of visible green color. Test FC and CYA together and evaluate them as a ratio, not as independent numbers.
Phosphates are plant nutrients — they feed algae directly. Sources include leaves, bird droppings, lawn fertilizer runoff, some pool chemicals (certain algaecides and clarifiers), fill water in some municipalities, and swimmer body products. High phosphate levels don't cause algae on their own, but they allow algae to grow more vigorously when chlorine momentarily drops.
Target: phosphates below 100 ppb. Above 500 ppb, algae treatment resistance increases and chlorine demand escalates. Test phosphates monthly in summer using a drop kit or photometer. If levels are consistently above 500 ppb, use a phosphate remover (Natural Chemistry Phosfree, BioGuard Back Up 2, Lo-Chlor Starver) per label before adding routine maintenance chemical.
Phosphate removers work by precipitating phosphates out of solution — they create cloudiness temporarily. Add before a visit, not at the end of one. The filter will clear the precipitate within 24 hours.
Algae establishes on surfaces first, then releases into the water column. The early-stage biofilm is invisible — you're brushing it off before you can see it. Weekly brushing of walls, steps, corners, and behind ladders disrupts the attachment phase and keeps algae from gaining a protected substrate.
Brushing technique matters:
A weekly dose of polyquat algaecide (Polyquat 60, BioGuard Banish) during peak summer months provides an additional layer of protection when FC fluctuates between visits. Polyquat works differently than chlorine — it disrupts algae cell membranes rather than oxidizing them. This means it can provide protection even when FC has temporarily dropped.
Key rules for algaecide use:
A dirty or underperforming filter doesn't remove algae spores and particulates efficiently. The filtration system is the physical removal component that works alongside chlorine's chemical kill. Both must be operating correctly for full protection.
| Filter Type | Summer Action | Frequency |
|---|---|---|
| Sand filter | Backwash when pressure rises 8–10 psi above baseline | Weekly or more |
| DE filter | Backwash + recharge DE after heavy storm or visible algae | Monthly or as needed |
| Cartridge filter | Rinse cartridge with hose (no soaking until end of season) | Every 2–4 weeks |
Log starting pressure at the beginning of each season in PoolLens. A filter running 8+ psi above baseline needs service — not just backwash, but inspection of the media condition and filter housing.
Catch algae before it becomes visible green water:
Mustard algae and black algae require different treatment than green algae. Standard shocking that clears green algae won't eliminate mustard or black algae. Mustard algae requires removal of all pool equipment (vacuums, brushes, toys) and simultaneous treatment. Black algae requires physical chipping with a stainless brush before chemical treatment can reach the root structure.
Track FC:CYA ratio, phosphate readings, algaecide additions, and brushing notes per visit. Identify which accounts are trend-watching or algae-prone before problems develop.
Open PoolLens Free →Maintaining adequate free chlorine relative to CYA level. The minimum FC is 7.5% of the CYA level — drop below this and algae gains a foothold within hours in warm water.
Yes. Brushing disrupts algae that has attached to surfaces before it becomes visible as a bloom. Weekly brushing in corners and steps removes biofilm while chlorine can still oxidize it.
A green pool can develop within 24–48 hours in summer conditions if free chlorine drops to zero. Algae populations double every 2–4 hours in warm water at peak growth conditions.
No. Algaecide is a preventive supplement, not a primary sanitizer. It slows algae growth but cannot substitute for adequate free chlorine. A pool with zero FC will turn green regardless of algaecide dosing.