Identify Your Algae Type First
Treatment approach changes significantly by algae type. Using the wrong method means wasted chemicals and a pool that clears briefly then comes right back. Know what you're dealing with before you add anything.
Green Algae
Water turns green — sometimes overnight. Free-floating or coating walls. Usually triggered by low free chlorine or missed shock. The most common and easiest to kill.
Easiest to treatYellow / Mustard Algae
Yellowish-brown patches on walls and floor, especially in shaded areas. Brushes off easily but comes right back. Chlorine-resistant — needs specific algaecide plus shock.
Moderately difficultBlack Algae
Dark blue-green or black spots embedded in plaster or concrete. Has a protective waxy layer that resists chlorine. Extremely stubborn — requires stainless steel brush and repeated treatment.
Hardest to treat🔍 Not sure which algae type you have? Describe your pool's symptoms and PoolDiag will identify it and give you a custom treatment plan.
Diagnose My Pool →Green Algae Treatment: Step by Step
Green algae is the most common pool problem in summer. It's caused by free chlorine dropping too low — either from a missed shock, heavy rain, high bather load, or CYA that's too high. The fix is aggressive but straightforward.
Test the water — know your baseline
Check free chlorine (FC), combined chlorine (CC), pH, alkalinity, and CYA. You need to know pH before shocking (it must be 7.2–7.4), and CYA to understand why your chlorine isn't holding. If CYA is above 80 ppm, see the CYA guide first.
Lower pH to 7.2–7.4
Chlorine at pH 7.2 is about 3× more effective than chlorine at pH 7.8. If pH is high, add muriatic acid first. Don't waste expensive shock on water that can't use it effectively.
Brush every surface thoroughly
Brush walls, floor, steps, corners, and behind the ladder. Break up algae colonies before chemical contact — physically disrupting the algae structure makes the shock penetrate far more effectively.
Triple shock at dusk
Add 2–3 lbs of cal-hypo per 10,000 gallons. Pre-dissolve in a 5-gallon bucket of pool water before adding. Pour around the pool perimeter. Always add at dusk — UV destroys chlorine in direct sunlight.
Run pump and filter 24/7
Keep the filter running continuously throughout treatment. Dead algae and debris are captured by the filter — this is how the water clears. Don't let the pump cycle off.
Backwash every 12 hours
Dead algae clogs filters fast. Backwash (or rinse the cartridge) every 12 hours while clearing. If filter pressure rises more than 8–10 psi above normal, backwash immediately.
Retest at 24 hours — repeat if needed
If FC has dropped near zero and water is still green or cloudy blue-green, the algae consumed all the chlorine. Add another full shock dose. Expect 2–4 treatments for heavy algae blooms.
Vacuum dead algae to waste
Once the water turns from green to cloudy-blue, vacuum the pool floor. Set your multiport valve to Waste (not Filter) so dead algae goes out of the pool rather than back through the filter.
Yellow / Mustard Algae Treatment
Mustard algae is frustrating because regular shocking barely phases it. It looks like sand or pollen on pool surfaces, and it recontaminates pool equipment — meaning brushes, toys, and even swimsuits need to be treated or it'll return within days of clearing.
Remove and treat all equipment
Take every brush, net, toy, float, and ladder out of the pool. Wash them separately with a diluted bleach solution (1 cup bleach per gallon of water). Wash swimsuits that were in the pool. Any untreated item reintroduced will restart the infestation.
Brush all surfaces aggressively
Brush every inch — walls, floor, steps, corners. Mustard algae grips surfaces in shaded spots. Brush before adding any chemicals.
Add a mustard algaecide
Use an algaecide specifically labeled for mustard or yellow algae — not a generic quat-based product. Follow the label dose. Add it before shocking.
Triple shock at dusk
Add 2–3 lbs cal-hypo per 10,000 gallons, pre-dissolved in a bucket. The algaecide + shock combination is what makes this effective — algaecide alone won't clear it.
Filter 24/7 and backwash frequently
Run continuously. Backwash every 12 hours. Vacuum to waste once the floor is visible.
Repeat treatment in 5–7 days
Mustard algae is persistent. Even if the pool looks clear, repeat a maintenance dose of algaecide + shock after 5–7 days. Two full treatment cycles is the standard for mustard algae.
Black Algae Treatment
Black algae is the most stubborn pool problem you can face. It embeds deep into plaster, concrete, and grout, and has a protective waxy coating that chlorine cannot penetrate without physical disruption. Treatment takes weeks, not days, and you must be aggressive from the start.
Use a stainless steel brush only
A regular nylon pool brush won't break the protective coating. You need a stainless steel bristle brush to physically scrub the waxy layer off every black spot. Brush hard — multiple passes per spot.
Apply cal-hypo directly to affected spots
Turn the pump off. Sprinkle granular cal-hypo directly onto each black algae spot. Let it sit on the surface for 10 minutes so the concentrated chlorine penetrates the now-exposed algae cells.
Brush again immediately after
Brush the treated spots one more time before turning the pump back on. This works the chlorine deeper into the surface.
Triple shock the entire pool
Turn the pump on and add 3 lbs cal-hypo per 10,000 gallons. Pre-dissolve and pour around the perimeter at dusk.
Run filter 24/7 and backwash
Keep the filter running continuously. Backwash every 12–24 hours.
Repeat weekly until spots are gone
Brush + direct spot treatment + shock every 7 days until the black spots are completely gone. Black algae rarely clears in a single treatment — expect 3–6 weeks for a severe infestation.
Algae Shock Dose Reference
| Algae Type | 10,000 Gal | 15,000 Gal | 20,000 Gal | Notes |
|---|---|---|---|---|
| Green (light) | 2 lbs cal-hypo | 3 lbs | 4 lbs | Add at dusk; brush first |
| Green (dark / heavy) | 3 lbs cal-hypo | 4.5 lbs | 6 lbs | May need 2–3 treatments |
| Yellow / Mustard | 3 lbs cal-hypo + algaecide | 4.5 lbs + algaecide | 6 lbs + algaecide | Treat equipment; repeat in 5–7 days |
| Black Algae | 3+ lbs cal-hypo | 4.5+ lbs | 6+ lbs | Spot treat first; repeat weekly |
🧪 Want an exact dose calculated for your pool's volume and current FC level?
Calculate My Dose →Algae Prevention
Once the pool is clear, the goal is to never let algae get a foothold again. Algae grows because chlorine dropped — usually because of one or more of these factors: