Green pool water is not just an aesthetic problem. It means algae has established itself in your pool and is actively competing with your sanitizer for control of the water chemistry. The fix is straightforward but it has to be done in the right sequence, at the right dose, and with the right follow-through. This guide gives you the complete protocol.
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Products for Algae Treatment at PoolPartsToGo: The shock that kills algae: Premium Pool Shock, 1 lb x 24 Bags ($109.99) The brush that exposes algae: Pool Brushes at PoolPartsToGo The cleaner that removes the dead algae: Pool Cleaners at PoolPartsToGo |
Why Your Pool Turned Green
Algae is present in every pool, in trace quantities, all the time. It enters via rain, wind, swimmers, and contaminated equipment. Under normal conditions, free chlorine at 1 to 3 ppm prevents algae from multiplying by killing cells faster than they reproduce. Green water appears when that balance breaks: free chlorine drops too low, algae multiplication exceeds the sanitizer's kill rate, and an established colony forms on pool surfaces and throughout the water column.
The most common causes of algae bloom in residential pools are straightforward: a week or more without shock treatment during warm weather, heavy rain that dilutes chlorine and introduces nitrogen compounds, a mechanical failure (pump, filter, or chlorinator not running), or CYA levels high enough to render existing chlorine ineffective regardless of the reading on the test strip.
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The Fastest Route to Green Water: CYA Above 70 ppm If your pool has been maintained with Dichlor or Trichlor tablets all season, CYA has been accumulating with every dose. CYA above 70 to 80 ppm binds with free chlorine and prevents it from reacting with algae, even when your test strip shows 2 to 3 ppm of free chlorine. The chlorine is there but it cannot work. This condition is called chlorine lock and it is why some pools turn green despite the owner having added chlorine regularly. If this describes your pool, a partial drain (25 to 30%) is required before any shock treatment will be effective. Test your CYA before treating. |
Diagnose Your Green Pool First
The shock dose required to clear a green pool depends directly on how severe the algae bloom is. Under-dosing is the most common reason algae treatments fail. Pool owners apply a standard single-bag maintenance dose to a Level 3 bloom and wonder why the pool is still green two days later.
Before calculating your shock dose, classify your pool's current condition against the scale below. The primary diagnostic is whether you can see the pool floor:
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Level |
What You See |
Can You See the Bottom? |
Shock Dose (per 10K gal) |
Expected Clearance |
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Level 1 Light Green |
Water has a greenish tint or haze. Pool looks cloudy but not opaque. |
Yes, clearly |
2 to 3 lbs |
12 to 24 hours with continuous pump operation. |
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Level 2 Medium Green |
Clearly green water. Visibility reduced. Walls and floor visible but murky. |
Partially (murky) |
3 to 5 lbs |
24 to 48 hours with multiple brushing sessions. |
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Level 3 Dark Green |
Pool is opaque green. Cannot see the floor at all. Algae visible on walls. |
No |
5 to 7 lbs |
48 to 72 hours with aggressive brushing and filter maintenance. |
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Level 4 Black-Green |
Dark green to nearly black. Severe algae bloom. Possible mustard algae on walls. |
No |
7 to 10 lbs + algaecide |
72 to 96 hours. May require backwashing multiple times and repeat treatment. |
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The Floor Visibility Test Is the Most Reliable Indicator If you can see the floor clearly, you have a Level 1 bloom that is still in early stages. If you can see the floor but only with difficulty through murky water, Level 2. If the floor is completely invisible, you are at Level 3 or 4 depending on whether the water is dark green or has a black-green appearance. Level 4 pools often also show visible algae mats on the walls near the waterline or at the steps. |
The Complete 9-Step Treatment Protocol
Every step matters. Skipping the pH adjustment makes the shock 30 to 50 percent less effective. Skipping the pre-treatment brush means the shock works on the biofilm coating the algae instead of the algae cells themselves. Skipping the repeat brushing at Hour 8 and Hour 24 allows dead algae to re-settle and decompose on the floor instead of being captured by the filter.
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1 |
Test pH first Before adding any shock, test and adjust pH to 7.2 to 7.4. Cal Hypo shock is significantly less effective at pH above 7.6. Add muriatic acid if needed and wait 2 hours before shocking. This one step can cut required shock dose in half. |
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2 |
Determine your algae level and calculate dose Use the severity table above to classify your algae bloom (Level 1 through 4). Calculate dose: Level 1 = 2 to 3 lbs per 10,000 gallons. Level 2 = 3 to 5 lbs. Level 3 = 5 to 7 lbs. Level 4 = 7 to 10 lbs. When uncertain, use the higher end of the range. Under-dosing is the most common reason a first treatment fails. Key: For a 15,000-gallon pool at Level 2: 3 to 5 lbs x 1.5 = 4.5 to 7.5 lbs. Round up to 5 lbs minimum. |
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3 |
Clean the filter before treatment Backwash your sand filter or rinse your cartridge to a clean baseline. A dirty filter restricts flow and cannot capture the surge of dead algae particles the shock treatment will produce. Start treatment with maximum filter capacity available. |
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4 |
Brush the entire pool, prioritizing steps and corners Brush all walls top to bottom, all step risers (including underside of lips), all corners and coves, behind ladders, waterline, and floor. This disrupts the biofilm protecting algae cells and exposes them to direct chemical contact. Shock applied to an unbrushed pool is significantly less effective. |
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5 |
Broadcast shock at dusk while walking the perimeter Open each bag and broadcast the granules evenly across the pool surface as you walk around the edge. Do not dump everything in one spot. Concentrate slightly more in the deep end. Do not pre-dissolve in a bucket. Set pump to its highest speed setting before adding shock. Key: Add shock at dusk or after dark. UV destroys unstabilized Cal Hypo rapidly. Evening application gives the chlorine overnight to work before sun exposure. |
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6 |
Run pump at high speed continuously Do not turn the pump off during treatment. Set it to high speed and leave it running. The pump circulates the shock through the entire pool volume and drives water through the filter where dead algae is captured. Any period of no circulation during treatment allows settling and re-establishment. |
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7 |
Brush again at Hour 8 and Hour 24 Return to the pool at 8 hours after shock and brush all surfaces again. Dead algae settled on the floor and walls during the night needs to be re-suspended for the filter to capture it. Brush a third time at 24 hours. Each brushing session also exposes any surviving algae cells to the residual free chlorine. |
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8 |
Clean filter every 8 to 12 hours The filter will fill with dead algae faster than normal. Check pressure every 8 to 12 hours. When pressure rises 5 or more psi, backwash or rinse the filter immediately. Restricted flow from a full filter slows treatment and may allow algae to re-establish in low-circulation zones. |
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9 |
Test and rebalance when water clears When water appears clear or light blue-gray, test free chlorine (must be below 3 ppm before swimming), combined chlorine (target zero), pH, alkalinity, and CYA. Add algaecide as a preventive measure. Wait until free chlorine is below 3 ppm before allowing any swimming. |
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Why Cal Hypo Is the Correct Shock for Algae Treatment Calcium hypochlorite (Cal Hypo) at 68% concentration is the correct shock product for algae treatment. It is stabilizer-free, meaning it does not add CYA to a pool that may already be at risk of chlorine lock. It delivers the highest chlorine concentration per pound of any shock product, which is why the dose tables in this guide are calibrated to 68% Cal Hypo specifically. Products sold at lower concentrations (such as 35% Cal Hypo available at big box stores) require significantly more product to achieve the same free chlorine level. The 24-count box at PoolPartsToGo provides enough full-strength bags for a severe algae treatment plus the follow-up maintenance schedule. |
The Complete Algae Brushing Guide
Brushing is not just support for the chemical treatment. It is a required step that determines whether the chemical treatment works at all. Algae in pools exists in a biofilm: a structured colony of cells attached to pool surfaces and coated in a protective polysaccharide matrix. The shock has to penetrate this matrix to reach and kill the algae cells. Brushing breaks the biofilm and exposes the underlying cells to direct chemical contact.
For algae treatment specifically, brush before shocking and then repeat every 8 to 12 hours until the pool is clear. Each brushing session disrupts any surviving colonies that are beginning to re-establish and re-suspends settled dead algae for the filter to capture.
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Area |
Priority |
Why and How |
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Steps and stair risers |
First |
Algae establishes on steps before anywhere else because they have the lowest circulation and highest surface area relative to flow. Use a 12-inch 360-degree brush. Clean all four sides of each riser, including the underside of the lip. |
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Corners and coves |
Second |
Where walls meet the floor is the lowest-circulation area in the pool. Algae colonies that survive chemical treatment most frequently restart here. Short scrubbing strokes with a 12-inch brush into the corner groove. |
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Walls (full surface) |
Third |
Overlapping horizontal strokes from top to bottom, pushing debris downward toward the floor and eventually the main drain. Use an 18-inch brush for efficient coverage of large wall sections. |
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Behind ladders and fittings |
Fourth |
Dead zones behind ladder uprights and around return fittings have no circulation and are prime re-establishment zones. Angle the brush into the shadow areas of each fitting. |
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Waterline |
Fifth |
Algae at the waterline is often anchored in organic residue from sunscreen and oils. Scrub at and just below the waterline on every session. |
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Floor |
Sixth |
Brush floor from the shallow end toward the deep end, directing debris toward the main drain. During algae treatment, suspended dead algae from brushing is what the filter captures. Brush, then let the pump run 2 hours before brushing again. |
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Brush Before Every Shock Dose, Not Just the First If your pool requires two or three shock treatments over multiple days, brush before each application. The 30 to 40 minutes of brushing before each shock treatment dramatically improves the effectiveness of every dose applied. Pool owners who skip this step and simply add more shock are often frustrated that the pool stays green despite the volume of product added. |
After the Water Clears: Removing Dead Algae
Clearing the water from green to blue-gray does not mean the treatment is complete. Dead algae has two places to go: the filter (if it is still suspended in the water column) and the pool floor (if it has settled). A pool that looks almost clear from the deck can have a thick layer of dead algae sediment on the floor that will cloud the water again if disturbed without being vacuumed.
After the shock treatment has cleared the visible green color, the final step is removing the settled dead algae from the floor. A robotic pool cleaner handles this more efficiently than a manual vacuum because it systematically covers the entire floor and captures debris in its internal filtration chamber without adding load to the pool filter.
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Post-Clearance Protocol: Removing Dead Algae From the Floor
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When Shock Is Not Working
The majority of failed algae treatments share one of six root causes. If your pool has not responded to shock treatment within 48 hours, identify which of these applies before adding more product:
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Symptom |
Most Likely Cause |
Fix |
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Pool stays green after 48 hours despite multiple shocks |
CYA above 70 ppm (chlorine lock). Chlorine tests show it is present but cannot sanitize. |
Test CYA. If above 70 ppm, partial drain (25 to 30%) is the only remedy. Refill, rebalance, then re-shock with Cal Hypo. |
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Water turns green again within a few days of clearing |
Algae was not fully eliminated. Surviving spores re-establish when chlorine drops. |
Complete a follow-up shock treatment 48 hours after clearing. Add algaecide. Maintain free chlorine above 2 ppm for 7 days after treatment. |
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Shock seems to disappear immediately (FC drops to zero within hours) |
Organic load is too high for the dose used. Very heavy algae bloom consuming chlorine faster than it can accumulate. |
Double the dose. Shock twice in one day: once in the morning, once in the evening. Run filter at high speed continuously. |
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Yellow or mustard-colored algae on walls that resists treatment |
Mustard algae (Cladophora) requires a different approach than green algae. |
Use a mustard algae-specific algaecide. Brush walls aggressively before and after treatment. Shock at triple dose. Clean or replace all equipment that contacted pool water (brushes, toys, etc.). |
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Pool clears but floor has a cloud of fine white or gray sediment |
Dead algae settled on the floor. Filter and pump cannot remove what has settled. |
Run robotic cleaner or manual vacuum to collect settled debris. Run pump at high speed after vacuuming to circulate and filter the suspended particles. |
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Filter pressure keeps spiking even after backwashing |
Heavy algae bloom is overwhelming filter capacity. Sand or DE filter may need DE powder added. |
Backwash or clean every 8 to 12 hours during treatment. For sand filters: add DE powder to skimmer (1 lb per 25 sq ft of filter area) to improve capture of algae particles. |
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The Most Common Reason: CYA Chlorine Lock If a pool has been maintained with stabilized chlorine products (Dichlor, Trichlor tablets) for a full season or multiple seasons without a partial water change, CYA is almost certainly above 70 ppm. This is the most common reason green pool shock treatments fail despite the pool owner adding the correct dose. Test your CYA before your next treatment. If it is above 70 ppm, no amount of Cal Hypo shock will clear the pool without a partial drain first. The correct sequence: drain 25 to 30 percent, refill, retest CYA (should be below 50 ppm), then begin shock treatment. |
The Shock That Clears Green Pools
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Blue Torrent Power Shock (68% Cal Hypo, 24 Bags) $109.99 (was $189.99) Premium Swimming Pool Shock - 1 lb Packs (24 Count) Full-strength 68% calcium hypochlorite in pre-measured 1 lb bags, 24 bags per box. The dose tables in this article are calibrated to this product's concentration. For a Level 2 algae bloom on a 10,000-gallon pool, use 3 to 5 bags. For a Level 3 bloom, use 5 to 7 bags. Stabilizer-free formula does not add CYA to a pool at risk of chlorine lock. Apply directly to pool water at dusk with pump at high speed. Do not pre-dissolve. Chemicals are non-refundable. |
Frequently Asked Questions
How many bags of shock do I need for a green pool?
It depends on your pool volume and algae severity. For a 10,000-gallon pool: Level 1 (light green tint) needs 2 to 3 bags. Level 2 (clearly green, murky) needs 3 to 5 bags. Level 3 (opaque green, cannot see floor) needs 5 to 7 bags. Level 4 (dark green to black) needs 7 to 10 bags. Scale up proportionally for larger pools: a 15,000-gallon pool at Level 2 needs 4.5 to 7.5 bags. Under-dosing is the most common failure mode. When uncertain, use the higher end of the range.
How long does it take to clear a green pool?
Level 1 pools (light green haze): 12 to 24 hours with continuous pump operation and one brushing session. Level 2 pools: 24 to 48 hours with two to three brushing sessions. Level 3 pools: 48 to 72 hours, may require a second shock dose. Level 4 pools (dark green): 72 to 96 hours minimum, with two to three shock treatments and frequent filter cleaning. These timelines assume continuous high-speed pump operation, correct pH before shocking, and proper brushing at each session.
Can I swim in my pool while treating algae?
No. During algae treatment, free chlorine is elevated significantly above safe swimming levels. The pool also contains dead and dying algae and the chemical compounds released during oxidation. Do not allow anyone to swim until free chlorine has dropped below 3 ppm, the water is visually clear, and combined chlorine tests at or near zero. After a heavy algae shock treatment, this typically takes 24 to 48 hours depending on weather, UV exposure, and pool volume.
Why is my pool still green after shocking?
The most common reasons, in order of frequency: CYA above 70 ppm causing chlorine lock (test CYA first), insufficient shock dose for the severity level (use the table above to recalculate), pH too high when shock was added (test and adjust before re-dosing), filter not cleaned frequently enough during treatment (clean every 8 to 12 hours), or the shock used was below 68% concentration (big-box store products often diluted). Identify which applies and correct it before adding more product.
Should I brush before or after adding shock?
Always before. Brushing disrupts the biofilm protecting algae cells and exposes them to direct chemical contact. Shocking into an unbrushed pool means the Cal Hypo is consumed by the organic biofilm coating rather than reaching the algae cells themselves. Brush every surface in priority order (steps and corners first, walls next, floor last), then add shock. Repeat brushing at 8 hours and 24 hours after the initial shock dose.
What is the difference between green and mustard algae?
Green algae (Chlorophyta) causes the classic green water bloom and responds to standard Cal Hypo shock treatment at the doses described in this article. Mustard algae (Cladophora) appears as yellow-brown deposits on pool walls that brush off easily but return rapidly. Mustard algae is chlorine-resistant and requires a mustard-specific algaecide in addition to triple-dose shock. It also survives on pool equipment, brushes, and toys that contact the pool water, which must be cleaned or discarded during treatment.
Your Pool Can Be Clear by Tomorrow
A green pool is fixable in 24 to 72 hours with the right dose, the right sequence, and the follow-through on brushing and filter maintenance. The 24-count box of 68% Cal Hypo shock at PoolPartsToGo provides enough product for a full algae treatment on most residential pools, plus the first few weeks of preventive maintenance once the pool is clear.
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Products at PoolPartsToGo: Premium Pool Shock, 1 lb x 24 Bags ($109.99) |
