A green pool is an algae bloom: the chlorine level dropped low enough, for long enough, that algae got ahead of it. It's not a mysterious event, and it's not usually the equipment's fault -- it's a chlorine demand that went unmet, most often after a run of hot days, heavy rain diluting the chlorine, or the pool just being left untested for a couple of weeks. The fix is a sequence, not a single product.
1. Test before you add anything
Get an actual reading on chlorine, pH and (if you can) phosphate before you dose anything. Dosing blind is how people end up over-chlorinating a pool that also has a pH problem, which wastes chemical and time. If pH is out (it usually is, sitting either too high or crashed), note it -- you'll rebalance after the shock, not before, since shocking will move pH again anyway.
2. Shock the pool -- meet the chlorine demand
"Shock" means dosing enough chlorine in one go to overwhelm the algae's chlorine demand, not just top up to a normal reading. A badly green pool can need several times a normal maintenance dose. Run the pump continuously while you do this and for the following 24-48 hours -- the point is to keep chlorine circulating everywhere the algae is, not just where you poured it in.
3. Check the equipment while it's clearing
Green water is also a good excuse to actually look at the gear: skimmer and pump baskets full of leaf litter reduce circulation and give algae a dead spot to hide in; a filter that's overdue a clean (see our cleaning-frequency guide for the pressure-gauge trigger) will struggle to pull out the dead algae once the chlorine's killed it.
4. Brush and vacuum
Dead algae still needs to be physically removed -- it settles as a fine silt on the floor and walls, especially in corners and steps (the pool's dead spots, where circulation is weakest). Brush the whole pool, including steps and ladders, then vacuum. Expect to do this more than once over several days as the water clears, not as a single pass.
5. Hold the normal schedule once it's clear
Once you're back to a clear pool with a normal chlorine reading, go back to testing 2-3 times a week rather than assuming it's fixed for good. Most green-pool relapses happen because testing stopped again the moment the water looked clear.
Black spot algae is a different problem
If what you're actually dealing with is dark, roughly circular spots stuck to the plaster or tiles rather than green, cloudy water, that's black spot algae -- a different species with a waxy outer layer that resists a normal chlorine dose. It needs a chlorine-based algaecide made for black spot specifically, plus physically brushing (sometimes acid-washing) the individual spots, since a general shock alone often won't shift it. Treating a black spot problem as a green-pool problem is a common reason people feel like they "fixed" the pool and it came straight back -- it's actually two different fixes for two different things.
Typical recovery
| Recovery cost | $250-$600 (DIY chemicals) if caught before it needs a professional clean-out |
|---|---|
| Time to clear | 3-7 days |
| Drain and refill needed | Rarely -- only for a very advanced bloom or a structural/equipment fault behind it |