Green pool, or a pool after a storm: getting it back

1Can you see the floor?

At a home pool the test is simple: can you clearly see the main drain? WA Health’s public-pool code sets clarity as a 150 mm Secchi disk on the bottom of the deepest part, visible from the concourse 9 m away.

Cloudy water shields algae and germs from the chlorine, so it works less well. Cloudy rather than green? See cloudy pool water.

2Test pH, stabiliser and free chlorine

pH. Bring it to 7.2–7.8 before you shock, so the chlorine works properly.

Stabiliser (cyanuric acid). It protects chlorine from the sun’s UV, but it also weakens it. That’s why stabilised pools need more free chlorine. WA Health’s public-pool code sets these minimums, and Healthy WA gives the same figures for home pools:

Keep stabiliser at or below about 50 mg/L (health guidance). It isn’t used up; only splash-out, backwashing or draining lowers it. Well above 50 mg/L? Partly drain and refill before you shock: high stabiliser means far more chlorine to clear the algae.

Shock. No Australian health code sets a shock level for algae, so dose to the product label. Keep swimmers out while you treat.

Salt pool? Shock it too: use the chlorinator’s boost or super-chlorinate mode (some manuals describe it for heavy use, debris or very hot weather), or liquid chlorine (AquaGUARD AquaFresh 1250) as the label directs. Healthy WA notes that some domestic chlorinators can’t keep up in heavy use. Weak cell? See salt chlorinator low output.

The SLAM method, in brief. Some techs use it on green pools:

• Shock, Level And Maintain: raise free chlorine to a shock level set from the stabiliser reading.
• Hold it there by testing and topping up, often several times a day, until the end points are met.
• The method’s own targets: water clear with no algae (dead or alive) on the floor or walls, combined chlorine 0.5 mg/L or lower, and no more than 1 mg/L of free chlorine lost overnight.
• Not an Australian standard. It’s a pool-owner and industry method from overseas. The product label dose is your baseline.
• The higher the stabiliser, the more chlorine it takes, because the shock level rises with stabiliser, and at very high stabiliser it becomes impractical. Bring it down first: see stabiliser too high.

Using an algaecide? Some manufacturers warn against copper-based algaecides in ionised pools, with some brands of chlorinator, or where copper may already be high. AquaGUARD Clear Guard is a non-copper algaecide. Follow the label.

3Brush, settle, vacuum to waste

• Brush the walls and floor to loosen the algae.
• Let the dead algae settle, then vacuum it off the floor. Don’t rely on the filter to catch it.
• Using floc (such as AquaGUARD Crystal Express)? Vacuum to waste, never through the filter. On a cartridge filter, take the element out first. Make sure you have a waste line before you start. Settling times vary by product, so follow the label.

Keep the filter running and clean. Run it continuously until the water clears, except while a floc settles with the pump off. Record the clean-filter pressure, and backwash or clean whenever it rises noticeably above that. Follow the filter manual.

Keep retesting and topping up chlorine. Still green after 24 hours? Shock again.

End-point checks

• Combined chlorine no more than 30% of free (WA public-pool code recommendation).
• pH, alkalinity and calcium hardness back in range before anyone swims. Use the LSI calculator.
• Swimmers back in only once total chlorine is within the WA Code’s in-use maximum of 8 mg/L, and at least 30 minutes after any dose (Healthy WA).
• Test for phosphate once the pool is clear, with a phosphate test strip or a Spin Touch analysis in store.

The WA Code covers public and commercial pools, not backyard pools, but it’s a useful benchmark.

After a storm or heavy rain

• Rain and run-off wash in dust, leaves, algae spores and phosphates, and can shift pH and alkalinity. Test; don’t assume.
• Floodwater reached the pump or electrics? A licensed electrician checks them before anything is switched on.
• Floodwater may have carried sewage in? Treat the pool as contaminated, not just dirty.
• Don’t pump a flooded pool out straight away. Emptying it can cause structural damage.
• Scoop and vacuum before running the filter (dirty water clogs it fast). Then retest, chlorinate as needed and run the filter overnight.
• Check the fence and gate are secure.
• Heavy rain forecast? Lower the water a little to cut overflow risk, but never below the skimmer, and never empty the pool. DWER’s WQPN 55 suggests about 25 cm of freeboard in the south-west for pools out of use for the season.

Getting rid of the water (WA)

Water Corporation doesn’t permit domestic pool backwash or drainage to sewer, and refers owners to their local council. The rest is from DWER’s Water Quality Protection Note 55:

• Keep it on the property (many WA councils require this by local law). Don’t run it to the road drain, parkland, wetlands or a waterway.
• Dechlorinate first if fully emptying: sodium thiosulphate, or leave the water in the sun.
• Salt water to a low-permeability evaporation pan where practicable; irrigation water must suit the plants.
• Discharge no faster than the soil soaks it up, with no overland flow.
• Not into septic tanks or leach drains: pool chemicals can kill the micro-organisms they need.
• Check with the council. Approval may be needed.

When it won’t clear

• No improvement from one day to the next. Poor circulation lets algae persist. If the pool is dosed with stabilised chlorine (trichlor tablets or dichlor granules), stabiliser builds up and may be higher than you think, slowing the chlorine. Retest it.
• The filter isn’t coping. See our media filter field guide.
• Marks that won’t brush off. It may be a stain, not algae. See pool stain identification.

Final step

Never mix pool chemicals, and keep chlorine away from acid. Use every product as its label and SDS direct.