Algae Green Pool Algae Green Pool — Do I Need to Drain My Green Pool? Step-by-Step Solution

Do I Need to Drain My Green Pool? Step-by-Step Solution

For informational purposes only. AI-assisted; may contain errors. full disclaimer ↓

Carol R.

First-time Pool Owner

Green pool - do I really need to drain it or are stores upselling?

Read full question

Despite my husband insisting our algae-covered pool "looks like a swamp that needs to be drained," I'm wondering if the pool store's costly advice is actually necessary. The local pool store is telling me I need to drain the whole thing and start fresh, which sounds incredibly expensive and wasteful. They're also trying to sell me a bunch of expensive chemicals and treatments.

I've been reading about DIY methods like the SLAM process but I'm not sure if that actually works or if I'm just wasting time. Has anyone successfully cleared a green pool without draining? I'm tired of throwing money at this problem and would love to know if there's a realistic way to fix this myself without the nuclear option of starting over.

Quick Answer

You typically don't need to drain a green pool. Most algae problems can be resolved using the SLAM (Shock Level And Maintain) method with proper chemical balancing and filtration.

When Draining IS Necessary

Before diving into the treatment process, let's identify the rare situations where draining might be your only option:

Black or extremely dark water - If you can't see the bottom at the shallow end
Structural damage visible - Cracks, staining, or deterioration that needs repair
Contamination issues - Sewage backup, chemical spills, or hazardous materials
Extremely high TDS - Very high TDS relative to your fill water (note: saltwater pools normally run high TDS, so TDS alone rarely makes balancing impossible)
Multiple failed treatment attempts - After 2-3 weeks of proper SLAM with no improvement

Step-by-Step Green Pool Recovery Without Draining

Step 1: Test Your Water Chemistry

Use a reliable test kit like the Taylor K-2006C to measure current levels
Test for Free Chlorine (FC), pH, Total Alkalinity (TA), Cyanuric Acid (CYA), and Calcium Hardness (CH)
Record all readings - you'll need baseline numbers to track progress
If CYA is over 100 ppm, consider partial draining to dilute stabilizer levels

Step 2: Calculate Your Shock Level

Determine your shock level based on CYA reading using the all-in-one pool calculator:
CYA 30-40 ppm: Shock level 12-16 FC
CYA 50 ppm: Shock level 20 FC
CYA 60-70 ppm: Shock level 24-28 FC
Calculate chlorine needed: 1 gallon of 12.5% liquid chlorine per 10,000 gallons raises FC by about 12-13 ppm (adjust proportionally for different concentrations - check your product label)

Step 3: Balance pH and Total Alkalinity

Lower pH to 7.2-7.4 using muriatic acid if above this range
Don't chase pH during the SLAM - high free chlorine makes the pH test read artificially high, so you can't measure it reliably anyway. Set it to ~7.2 before you start and leave it; liquid chlorine is roughly pH-neutral over a full cycle
Adjust Total Alkalinity to 80-120 ppm for regular pools (60-80 for salt water generators)
Add 1.5 lbs of sodium bicarbonate per 10,000 gallons to raise TA by 10 ppm

Step 4: Begin the SLAM Process

Add enough liquid chlorine to reach your calculated shock level
Run your pump and filter 24/7 throughout the entire process
Brush all pool surfaces thoroughly - walls, steps, corners, and behind ladders
Clean or backwash your filter when pressure rises 8-10 psi above clean pressure
For sand filters, backwash daily during heavy algae treatment

Step 5: Maintain Shock Level

Test FC levels every 2-4 hours during daylight
Add more liquid chlorine to maintain your shock level - algae consumes chlorine rapidly
Continue brushing twice daily to break up algae colonies
Don't add algaecides - the shock-level chlorine already kills the algae, and it just gets oxidized (wasting money), with polyquat types tending to foam

Step 6: Monitor Progress and Test for Completion

Water should begin clearing within 24-48 hours
Perform the Overnight Chlorine Loss Test (OCLT) once water is clear
Test FC levels before bed and again first thing in the morning
Loss should be 1 ppm or less overnight when algae is dead
Confirm combined chlorine (CC) is 0.5 ppm or less, and the overnight chlorine loss (OCLT) is 1 ppm or less, with the water clear, before ending SLAM (those are TFP's three exit criteria, all met at the same time)

Why Avoid Draining When Possible

Draining your pool creates several risks and expenses:

Structural damage - Pools can crack or "pop" out of the ground without water weight
Equipment damage - Pumps can run dry and burn out
Higher costs - Refilling costs $200-600 depending on size and water rates
Chemical restart - You'll need to rebalance entirely new water
Time loss - Refilling and rebalancing takes 2-3 days minimum

Special Considerations

Sand Filter Users: Consider flocculant (floccing) if your sand filter can't clear fine particles. Add about 4 lbs of aluminum sulfate per 10,000 gallons (alum is dosed in pounds, not ounces — roughly 2-8 lbs depending on how cloudy the water is), let settle 24 hours, then vacuum to waste.

Cartridge Filter Users: Have spare cartridges ready - you'll need to clean or replace them frequently during treatment. Rotate between sets while cleaning.

DE Filter Users: Expect to break down and clean your filter multiple times. Don't add new DE until the algae is completely dead.

Timeline Expectations

Most green pools clear within 3-7 days using proper SLAM technique. Factors affecting timeline include:

Severity of algae bloom
Filter type and condition
Consistency in maintaining shock levels
Thoroughness of brushing
Starting chemical balance

Safety Warning: Always add chemicals to water, never water to chemicals. Wear safety goggles and gloves when handling pool chemicals. Ensure proper ventilation when using muriatic acid.

For the full breakdown of safe chlorine levels by CYA level, see our pool water chemistry guide.

Safety first: follow every product label and your equipment manual, wear protective gear (gloves and eye protection), and call a pro when a job is beyond you. safety details ↓

Recommended for this job