Algae in a pool is not one problem with one solution. It is three biologically distinct organisms (chlorophyta, mustard algae, and cyanobacteria) that look different, attach differently, and respond to completely different chemical regimens. Owners who treat all three the same way waste hundreds of dollars in chemicals and watch the pool stay tinted for weeks. Owners who identify the species first and match the treatment finish in days. This guide walks through identification, dosing, and the realistic cure timeline for each of the three.
Green algae: the common case
Green algae (chlorophyta) is the algae 90 percent of pool owners encounter. It blooms when free chlorine drops below 1 ppm, when CYA climbs above 80 ppm, or after a heavy rain dumps phosphates into the pool. The water turns from a slight yellow-green haze to a full pea-soup green over 24 to 72 hours.
Identification: uniform green tint throughout the water column. Visibility drops from clear (3 meters) to blurred (1 meter) within a day. Walls may feel slippery to the touch but algae does not cling tightly.
Treatment sequence (3 to 5 day cure):
- Brush all surfaces (walls, floor, steps, behind ladders, light niches) thoroughly to suspend algae in the water column. A 25 cm nylon-bristle brush handles vinyl and fiberglass. A stainless steel brush is required for plaster but will tear a vinyl liner.
- Test and balance pH to 7.4 to 7.6 first. Chlorine works far better at correct pH.
- Triple-shock with calcium hypochlorite at 30 grams per 10000 liters. Dose at dusk to prevent UV burn-off. Run the pump for 24 hours continuously after dosing.
- Re-test free chlorine the next morning. Should read 6 to 10 ppm. If FC has dropped below 4 ppm, repeat the shock dose.
- Day 2: brush again, vacuum any settled algae on the floor (use waste setting if available).
- Day 3: chlorine demand should drop. Free chlorine holds steady at 4 to 6 ppm after a daytime cycle. Water should appear hazy gray (dead algae suspended) rather than green.
- Day 4 to 5: run the filter continuously. Backwash sand filters or clean cartridges. Water clears progressively. Add a maintenance dose of polyquat 60 percent algaecide on day 4 to prevent rebloom.
Total chemical cost: 60 to 150 dollars for a typical 50000 liter pool. Severe swamp-green pools can take 7 to 10 days and 200 to 400 dollars of chemicals if biofilm has built up.
Yellow (mustard) algae: the chlorine-resistant case
Mustard algae looks like fine yellow or tan dust that settles on shaded walls, behind ladders, and on the deep-end floor. It does not cloud the water. Owners often mistake it for sand, pollen, or dirt because it brushes away easily and then returns within hours.
Identification: yellow-tan dust on walls and floor, particularly in shaded areas. Brushes away in a cloud but resettles in 4 to 8 hours. Free chlorine readings stay normal (2 to 4 ppm) even during an active bloom, which is the giveaway.
Mustard algae is naturally chlorine-resistant because its cell walls block hypochlorous acid penetration. Standard shock doses kill the suspended cells but spore-stage cells on surfaces survive and rebloom within 24 hours.
Treatment sequence (5 to 7 day cure):
- Remove all swimwear, pool toys, floats, robotic cleaner, and brushes from the pool. Wash them in a separate basin with 5 ppm chlorine bleach water for 30 minutes. Mustard algae survives on these surfaces and reinfects after each clear.
- Brush every surface vigorously. Pay extra attention to shaded corners and behind ladders.
- Triple-shock with cal-hypo plus a sodium bromide booster (sold as mustard algae treatment, 30 to 50 dollars per bottle). The bromide converts in the pool to hypobromous acid, which penetrates mustard algae cell walls.
- Brush daily for 4 days. Re-shock on day 3 with a half dose.
- Day 5 to 7: free chlorine should hold steady, walls should stay clean for 24 hours, and the yellow dust should not reappear after brushing.
Total chemical cost: 80 to 200 dollars. The key cost is the sodium bromide treatment. Skipping it and running cal-hypo alone is the most common mustard algae failure.
Black algae: the embedded case
Black algae (cyanobacteria) is the hardest pool algae to treat. It looks like black spots, the size of a small coin, embedded into plaster or grouting. Each spot is a colony with a hard outer shell and microscopic roots that grow into porous pool surfaces. Pure chlorine cannot reach the cells inside the shell. Mechanical disruption is required first.
Identification: discrete black spots on plaster, gunite, or grout. Spots have a hard outer layer that does not brush off easily. Vinyl liners do not get black algae (the surface is not porous enough), but black algae in a vinyl pool is sometimes confused with stain or metal deposit.
Treatment sequence (2 to 4 week cure):
- Brush each spot individually with a stainless steel brush. Brush hard enough to break the outer shell and expose the cells underneath. This is the most important step. Skipping it makes chemistry useless.
- Apply a copper-based algaecide directly to the brushed spots (squirt-bottle a concentrated copper sulfate solution onto each spot, with the pump off, and let it sit for 60 minutes).
- Restart pump and triple-shock the pool with cal-hypo at 30 grams per 10000 liters.
- Repeat brushing and direct-application copper treatment every 3 days for 2 weeks.
- Monitor for new spot growth. Black algae roots can survive a single treatment and regrow from a single missed cell.
Total chemical cost: 150 to 300 dollars. Plan for a follow-up plaster cleaning at season end. Black algae permanently stains plaster and the only complete removal is acid-washing during winter closing.
Prevention
The cheapest algae treatment is the one you never have to do. Hold free chlorine above 1 ppm at all times. Keep CYA below 50 ppm (do partial water replacement annually if you use chlorine tablets). Brush walls weekly even when the pool looks clean. Run a phosphate remover monthly in areas with high fertilizer runoff. Run the pump 8 to 12 hours per day during summer. A pool on these basics rarely sees algae and saves 200 to 400 dollars per season in reactive chemistry.
Frequently asked questions
How can I tell green algae from cloudy water?+
Green algae tints the water a uniform pale to dark green and reduces visibility from the deck. Cloudy water (chemical imbalance, fine debris, or post-shock haze) is white, gray, or milky and does not have a green tint. If a white pool surface (like a plaster step or skimmer faceplate) appears tinted under the water line, you have algae. If it just appears blurry but still white, you have cloudy water.
Why does my pool turn green every summer despite chlorine?+
Recurring algae usually means cyanuric acid (stabilizer) is too high, which makes free chlorine progressively less effective even when test readings look fine. Above 80 ppm CYA, you need a free chlorine level of 6 to 8 ppm to match the disinfecting power of 2 ppm at 30 ppm CYA. The fix is partial water replacement to drop CYA below 50 ppm. Phosphate buildup from rainwater and fertilizer runoff also feeds recurring blooms.
Will pool shock alone kill algae?+
Yes for green algae, no for yellow or black. Green algae is suspended in the water column and dies under a sustained 4 to 6 ppm free chlorine for 24 hours, achievable with a single triple shock dose. Yellow (mustard) algae is chlorine-resistant and clings to walls behind brushes, requiring brushing plus shock plus a specialty algaecide. Black algae embeds roots into plaster and requires mechanical removal (stainless steel brushing) before chemistry can reach the cells.
Are algaecides necessary or is chlorine enough?+
For routine green algae prevention, chlorine alone is enough if free chlorine is held above 1 ppm and CYA is below 50 ppm. For active green blooms, chlorine alone clears them but a polyquat 60 percent algaecide added at the end speeds clearing by 24 hours. For yellow and black algae, specialty algaecides (sodium bromide for yellow, copper-based for black) are required because pure chlorine cannot penetrate the cell walls of those species.
How much does it cost to treat a green pool?+
A typical green pool treatment costs 60 to 150 dollars in chemicals for a 50000 liter residential pool in 2026. The cost is mostly cal-hypo shock (3 to 5 bags at 12 to 15 dollars each) and a polyquat algaecide bottle at 30 to 50 dollars. A severe swamp-green pool can run 200 to 400 dollars and 7 to 10 days of treatment if biofilm has built up on the walls. Yellow algae averages 80 to 200 dollars. Black algae averages 150 to 300 dollars plus 2 to 4 weeks of treatment.
