A pool can go green under a cover because algae can still grow when the water is warm and sanitiser is not keeping up. That is a common explanation, but colour alone does not establish the cause. A blanket saves water and helps retain heat; it does not disinfect the pool. Removing it will not by itself clear the water.
Before adding anything, test the water. A green tint alone is not enough to work out a treatment dose, and repeating last summer's chemical routine may not suit the pool's current condition.
Questions answered in this guide
Can the cover cause algae?
It can contribute to the conditions algae like. WA Health explains that a covered pool can become warmer near the surface in summer, encouraging an algal bloom. Daisy Pool Covers identifies low chlorine, warm water and nutrients as factors behind green water.
That is why āI covered it, so I stopped maintaining itā can end badly. Keeping leaves out is helpful, but water still needs sanitising, circulation and testing. A blanket cannot take over those jobs.
Think back to what changed before the water turned. Was there a hot spell? Did the pump timer change? Was the pool left covered while you were away? Did the sanitiser run out? Those details help explain the change and are worth passing to the pool technician with the test results.
What should I do when I first notice green water?
Keep swimmers out while you investigate. Fully remove the cover so you can see the water and pool surfaces. If the water is cloudy enough to hide the bottom, visibility is another reason not to swim.
Then work through the problem in this order:
- Measure, rather than guess. Test free chlorine and pH with a suitable kit. Take a fresh sample to a pool shop for a fuller test if the result is unclear, the water is very green or the problem keeps returning.
- Check whether the equipment is doing its normal job. Look at the pump schedule, skimmer basket and filter condition. Report any unusual noises, reduced flow or dosing warnings; do not compensate for a malfunction simply by adding more chemicals.
- Get a treatment plan for the actual pool. The technician needs its volume, sanitising system, recent products added and test results. Write down the quantities, order, circulation requirements and retesting instructions.
- Remove the remaining debris and retest. Brushing and vacuuming may be part of the plan. Use the method appropriate to the pool and filter, rather than sending a large amount of debris somewhere the system cannot handle.
Do not mix pool chemicals together. Add them only as their labels and the treatment instructions require, with nobody in the water. A generic internet quantity cannot account for your pool's volume or the concentration of the product in your shed.
Should I put the cover back on while treating the pool?
Not during super-chlorination. SPASA advises leaving the cover off during that treatment and until chlorine has returned to the appropriate level. Follow the chemical and blanket instructions for your particular products rather than treating a fixed number of hours as permission to cover or swim.
Store the rolled blanket as its maker directs, protected from sun when off the water.
How do I stop it happening again?
Keep a short record for the next couple of weeks: test results, treatment changes, unusually hot weather and how long the cover stays on. If the pool starts drifting out of balance again, that record is much more useful than finding another green pool at the weekend.
Ask the pool technician to review the normal pump and dosing routine for covered conditions. Do not assume the same dose is always required with the cover on and off. Equally, do not reduce treatment just because the water looked clear yesterday.
Continue removing the blanket for inspection even when nobody plans to swim. If handling is the obstacle, a suitable combination of pool cover and roller can make the job easier. Keep the testing kit where you can find it, and make looking at the water part of the routine rather than a surprise at the weekend.
When can we swim again?
When the water is clear, the treatment is complete and testing confirms it is suitable for swimming under the product instructions and your technician's advice. Colour improving is not the only test. Completely remove the blanket before anyone enters: a solar blanket is not a safety cover and does not replace compliant pool fencing.
