Green pool water almost always means one thing: chlorine has dropped low enough that algae took over. The good news is that even a fully green pool can be cleared in 3 to 5 days for under $150 in chemicals if you attack it in the right order. Do it in the wrong order — shocking before adjusting pH, for example — and you'll spend twice as much chemical and still end up cloudy. Here's the process every pool tech follows.
Not all green is the same, and the treatment differs. Look at the pool in daylight and match one of these:
Test pH, alkalinity, and chlorine with a fresh liquid test kit (test strips read badly on green water). Adjust pH to 7.2 to 7.4 BEFORE shocking — chlorine works up to 10x better at pH 7.2 than at 7.8. Use muriatic acid to bring pH down. Alkalinity should be 80 to 120 ppm.
Skim out as much visible debris as possible and brush the walls thoroughly. Algae clings to plaster and vinyl and won't die if you don't scrub it into suspension where the chlorine can reach it.
This is where most people fail. A single bag of pool shock will not fix a green pool. You need to reach 'breakpoint chlorination' — roughly 10 to 30 ppm free chlorine depending on how green the pool is. For a 15,000-gallon pool, that means 4 to 8 pounds of calcium hypochlorite (cal-hypo) shock, or 3 to 6 gallons of liquid chlorine, added at dusk.
Do not use stabilized chlorine (dichlor or trichlor) to shock — it dumps cyanuric acid into the water and locks up your chlorine for weeks. Cal-hypo or liquid only.
Run the pump 24 hours a day until the water clears. This is non-negotiable. Backwash a sand or DE filter every time pressure rises 8 to 10 psi over baseline; clean a cartridge filter daily. The pool will likely go from green to cloudy gray on day 2 — that's dead algae, which is progress.
Test chlorine morning and evening. Re-shock any time free chlorine drops below 5 ppm. Add a quality polymer clarifier or a flocculant on day 3 to bind up the fine dead algae particles so the filter can grab them.
Once water is clear, vacuum any settled dead algae to waste (not through the filter) so you don't push it right back into the pool. Rebalance chlorine to 2 to 4 ppm, pH to 7.4 to 7.6, alkalinity to 80 to 120, and calcium hardness to 200 to 400. Add a maintenance dose of algaecide as prevention. Do a full water sample test at the pool store within a week to confirm chemistry is holding.
The green pool you just fixed almost always comes back within 12 months if you don't address the underlying cause. In 80% of cases, the root cause is one of two things: cyanuric acid (stabilizer) is too high and locking up your chlorine, or phosphates in the water are feeding algae faster than chlorine can kill it.
Test cyanuric acid quarterly and keep it between 30 and 50 ppm. If it's over 80 ppm, the only fix is to partially drain and refill — chlorine simply doesn't work well above that level regardless of how much you add. This is why switching to stabilized chlorine tablets year-round quietly ruins pools; the stabilizer accumulates and never breaks down.
Phosphate remover ($30 to $60 per bottle) added twice a year strips the algae's food source and dramatically reduces chlorine demand. It's one of the cheapest and most underused prevention chemicals in the pool trade.
Some situations need professional gear or chemistry expertise: a pool that's been green for months (biofilm on plumbing), any sign of black algae, plaster that's turning gray-green under the algae (staining is starting), or a pool where you've done everything above twice and it's still green. Green-to-clean service typically runs $250 to $600 and includes chemicals — often cheaper than the third round of shock you'd buy yourself.
A light green pool with functioning equipment can clear in 24 to 48 hours. A moderately green pool takes 3 to 5 days. A dark forest-green pool with visible wall growth is a 5 to 7 day project, and severe cases with black algae can take 2 weeks. If it's been more than a week without visible improvement, something is off — usually pH out of range or a broken filter.
No. Wait until free chlorine drops back below 5 ppm — usually 24 hours after the last shock dose. Swimming in shocked water can cause skin irritation, ruined swimsuits, and bleached hair. Test chlorine before letting anyone in the water; don't just guess based on how the water looks.
One of three reasons: chlorine is chronically low (usually a stabilizer / cyanuric acid problem), phosphates in the water are feeding algae faster than chlorine can kill it, or the filter is undersized or clogged. If a properly shocked pool goes green again within a month, get a full water sample tested at a pool store — the fix is usually a specific chemical adjustment, not more chlorine.
Shocking with cal-hypo or liquid chlorine is not harmful to the pool — that's what the equipment is designed for. What's harmful is repeatedly using stabilized shock (dichlor or trichlor), which builds up cyanuric acid to the point where chlorine stops working. Rotate to unstabilized shock for cleanup work.
Almost never. Draining is a last resort reserved for pools with cyanuric acid over 100 ppm, extreme metal staining, or complete chemistry failure. A full drain risks the plaster shell popping out of the ground from hydrostatic pressure, especially in areas with a high water table. Try the green-to-clean process at least twice before considering a drain.