How to clear a green pool, and when to stop and call
Something stopped before the pool turned. Fix that first, then brush, shock and filter in the right order. And know when a drain is the cheaper answer.
A pool goes green for one reason: chlorine stopped keeping up. Something behind that stopped first, usually the pump, the filter or the chlorinator, and if you don't fix that part the pool will be green again in a month. Here is the order of operations, and the point where it's cheaper to call.
1. Find out what stopped
Is the pump running? Is the filter pressure normal, or pinned high? Is the chlorinator empty, or the salt cell showing a fault? Fix the equipment first. Shocking a pool with no circulation wastes the chlorine.
2. Test before you treat
Free chlorine, pH and stabilizer (cyanuric acid). Stabilizer decides the plan. If it's over about 100 ppm, chlorine can't work no matter how much you add, and the fix is a partial drain and refill. Years of tablets do this. If stabilizer is reasonable, you can treat the pool where it stands.
3. Brush, then shock
Brush the walls, steps and corners so the algae is in the water where chlorine can reach it. Then add liquid chlorine, not tablets. A rough starting dose for a light green pool is a gallon of 12.5 percent liquid chlorine per 10,000 gallons, added in the evening with the pump running. Test the next morning. If the chlorine is gone, the algae ate it, so dose again. You're done shocking when the water holds chlorine overnight.
4. Run the pump around the clock and clean the filter
Dead algae clogs the filter fast. Watch the pressure gauge and clean the cartridge or backwash whenever it climbs eight to ten psi over clean, which may be daily for a few days. If you have a multiport valve, vacuum the dead algae to waste so it never hits the filter at all. Gray, cloudy water after the green is gone is a filtration problem, not a chemical one. Keep filtering.
5. Expect a few days
Light green with working equipment clears in three to five days. Dark green takes a week or more. Black algae rooted in the plaster is a different problem that needs scrubbing and spot treatment, and a pool that sat swampy for months is often cheaper to drain and acid wash than to fight.
When to call
- The pump, filter or chlorinator isn't working. That's a repair before it's a cleanup.
- Stabilizer is over 100 ppm. Draining a pool safely takes more than a hose, especially with old plaster or a high water table.
- You can't see the bottom. That's a safety problem as much as a water problem.
- You've shocked twice and it's still green. Phosphates or stabilizer are probably why, and a test will say which.
- The algae is black, or the pool was green for months.
Once the water is clear, get the cause fixed, or you'll be reading this again next summer.