Between July and September, two things happen at once across Delhi NCR: pools get chemically hammered, and owners stop paying attention because nobody is swimming. That combination is why more pools turn green in monsoon than in peak summer.
What rain does to your water
- Dilutes free chlorine, sometimes to zero after a heavy night
- Lowers pH, since rainfall here is mildly acidic
- Drops total alkalinity, removing the buffer that keeps pH stable
- Adds phosphates and nitrates through runoff — direct algae food
- Raises water level, pushing the surface above skimmer intake so surface debris stops being collected
The monsoon checklist
After every heavy rainfall: test free chlorine and pH. Correct alkalinity first if it has fallen, then pH, then sanitiser. Lower the water back to mid-skimmer level.
Weekly: brush walls and floor even if the water looks fine — spores settle in still water. Clear skimmer and pump baskets, which fill far faster with wind-blown debris.
Keep the pump running. This is the single most important instruction of the season. Reduced hours plus diluted chlorine plus phosphate load equals a bloom within days.
Check phosphates if algae keeps returning. Runoff from surrounding lawns and planters is usually the source. A phosphate remover breaks the cycle where more chlorine will not.
Cover with care. A cover keeps debris out but traps organics and stops sunlight-driven off-gassing. If you cover, test more often, not less.
The equipment side
Damp plant rooms, voltage fluctuation and standing water around pumps are all monsoon hazards. Check electrical enclosures and drainage around the pump pad, and make sure the backwash line actually discharges away from the plant area.
Cheaper than recovery
A monsoon bloom means shock chemicals, repeated backwashing, days of downtime and sometimes a partial drain. Continuous cover through the rains costs a fraction of that. Book a free pool health check to get your monsoon baseline documented.