Why your Paphos pool turns green overnight and what science says about It.

Before anything else, it is worth knowing that a green pool can have three different causes, and treating the wrong one wastes time and money.
Cause 1: Algae the most common culprit. Algae are photosynthetic microorganisms that thrive when chlorine cannot kill them fast enough. The water turns green, sometimes with a cloudy or murky appearance.
Cause 2: Copper oxidation if your pool uses copper-based algaecide or has copper pipes or fittings, the metal can oxidise and turn water a vivid blue-green, even in a well-sanitised pool.
Cause 3: Water chemistry imbalance specifically, too much total alkalinity relative to calcium hardness can shift the LSI (more on this below) and cause the water to appear greenish or cloudy.
A simple test: fill a white bucket with pool water and add a small amount of liquid chlorine. If the water clears within 30 seconds, the cause is organic most likely algae. If it stays green, you are dealing with metals or a chemistry imbalance.

The CDC recommends maintaining free chlorine between 1–4 ppm with a pH of 7.2–7.8 to ensure effective disinfection. But this guidance assumes little or no cyanuric acid (CYA) in the water. In a stabilised outdoor pool which describes virtually every pool in Paphos — the relevant number is not the free chlorine reading alone. It is the FC:CYA ratio.
When CYA is present, the impact pH has on HOCl concentration is minimal, if not negligible. The ratio of free chlorine to CYA is more significant than pH in terms of attaining the concentration of HOCl required to kill pathogens and algae. In plain terms: a pool showing 3 ppm free chlorine with 80 ppm CYA has far less active sanitiser than a pool showing 2 ppm free chlorine with 30 ppm CYA.
The guideline is to maintain free chlorine at 7.5% of the CYA level. At 40 ppm CYA, that means keeping free chlorine at a minimum of 3 ppm. At 80 ppm CYA a level commonly found in Paphos pools maintained with stabilised chlorine tablets you need at least 6 ppm free chlorine to achieve the same disinfection. Most pools are not dosed anywhere near this. The CMAHC Ad-Hoc Committee for Cyanuric Acid has proposed the FC:CYA ratio as the new industry standard, replacing blanket CYA ppm ranges or flat free chlorine minimums.
UV radiation destroys unstabilised chlorine in hours. Paphos regularly sees a UV index of 10–11 during summer. Without cyanuric acid (CYA) as a stabiliser, up to 75–90% of free chlorine can degrade within two hours of direct sunlight exposure. This means a pool that tested fine at 8am may have virtually no active sanitiser by midday.
High temperatures accelerate all biological processes. With water temperatures reaching 28–32°C in Paphos pools during summer, the reproduction rate of algae and bacteria increases dramatically. According to Orenda Technologies, algae take over a pool when the growth and reproduction rate of contaminants exceeds the killing rate of the sanitiser. In summer heat, that threshold is reached faster.

CYA protects chlorine from UV but too much CYA becomes its own problem. CYA binds to HOCl and reduces its killing speed. The more CYA in the water, the weaker your chlorine becomes, even at the same ppm reading.
Orenda's guideline is to keep CYA below 50 ppm. Many pools in Paphos, particularly those maintained with stabilised chlorine tablets (trichlor or dichlor), accumulate CYA far beyond this level over time. The result: the total chlorine reading looks fine. The water even looks clear. But the actual concentration of active HOCl is so low that algae can reproduce faster than it is being killed.
This is one of the most common reasons a pool "suddenly" turns green  it has been slowly losing its defence for weeks.

Algae need two things to thrive: a reduced chlorine killing rate, and nutrients. The primary nutrient is phosphates.
Phosphates enter the pool through tap water, rain, dust, leaves, sunscreen, and even some pool chemicals. According to Orenda, the ideal phosphate level is below 500 ppb (parts per billion). Elevated phosphates do not directly "fight" chlorine but they allow algae to reproduce so rapidly that even healthy chlorine levels can struggle to keep up.
After a Sahara dust event which Paphos experiences regularly between spring and autumn phosphate levels in a pool can spike significantly within 24–48 hours. Combined with UV-degraded chlorine, this creates exactly the conditions for an overnight green pool.

Most pool services in Paphos check the colour of the water and adjust chlorine tablets accordingly. We work differently.
On every visit, we measure free chlorine and pH — and from these readings, combined with CYA, temperature, alkalinity, and calcium hardness, we calculate two things that actually determine whether your pool is safe and balanced:
Disinfection efficiency is assessed through HOCl — the active killing form of chlorine. A free chlorine reading alone tells you nothing without knowing the FC:CYA ratio and pH. We calculate the actual HOCl concentration in your water, not just the total chlorine number. This is what determines whether your pool can kill pathogens and suppress algae.
Water balance is assessed through the Langelier Saturation Index — a single calculated value that reflects the aggregate relationship between pH, alkalinity, calcium hardness, CYA, temperature, and TDS. The LSI tells us whether your water is corrosive to surfaces and equipment, balanced, or scaling. We dose to target a healthy LSI, not just acceptable ranges for individual parameters.
Every visit generates a report with these results that you can access online. Not to impress you with numbers — but because this is the only way to know, with certainty, that your pool is both safe to swim in and not silently damaging itself.
Crystal clear water is not evidence of safe water. A scientifically tested pool is.

If your pool has turned green, resist the urge to dump in chlorine tablets without testing first. Instead:

1. Test for CYA if it is above 60–70 ppm, partial water dilution may be needed before more stabilised chlorine will help.
2. Calculate your FC:CYA ratio — if it is below 5%, your chlorine is not effectively sanitising. For normal maintenance, target an FC:CYA ratio of around 7.5%. For shock treatment, raise FC to 30–40% of your CYA level using non-stabilised chlorine. For example, at 50 ppm CYA, a proper shock dose means bringing FC to 15–20 ppm. Partial water dilution may be needed if CYA has accumulated to a level where achieving these ratios is impractical.
3. Use non-stabilised chlorine
4. Brush the entire pool surface thoroughly — algae hide in biofilms on walls and floors where chlorine cannot easily reach.
5. Run filtration continuously until the water clears.

If the problem keeps returning, the root cause has not been addressed. We offer a diagnostic visit that identifies what is actually driving the issue in your specific pool.

Pool Health provides science-based pool maintenance in Paphos, Sea Caves and Pegeia, Cyprus. From education to advanced maintenance and installations. Book a visit.