This is the cause of green water in the majority of cases Pool Doctors Uganda is called to address. When free chlorine drops below 1.0 parts per million (ppm), your pool loses its primary defence against algae. Algae spores are present in every pool — introduced naturally through rain, wind, and swimmers. They are invisible and harmless when chlorine is at the correct level. When chlorine drops, those spores begin multiplying. Within 24 to 48 hours in Uganda’s warm temperatures, a clear pool can become visibly green.

The most common reason chlorine drops in Uganda is not that you stopped adding it — it is that the chlorine you are adding is being destroyed faster than you realise. Uganda’s direct equatorial sunlight breaks down unstabilised chlorine extremely rapidly. If you are using unstabilised chlorine in an outdoor pool, you may be adding the correct dose but losing it within hours of treatment.

This is the cause that surprises most pool owners. Your pool can have the correct chlorine level — test it and it reads 2.0 ppm, well within the safe range — and your pool will still turn green. How? Because if your pH is above 7.8, your chlorine becomes progressively less effective. At pH 8.0, chlorine is operating at roughly 20% of its normal effectiveness. At pH 8.5, it is essentially doing nothing.

In Uganda, pH tends to rise after heavy rain because rainwater is naturally slightly alkaline. It also rises when pool water evaporates in direct sunlight and is replaced with fresh water. A pool owner who adds chlorine consistently but never tests or adjusts pH will find their pool turning green despite what appears to be correct chemical treatment.

Your filter is your pool’s kidney. When it is working correctly, it circulates chemically treated water through every part of the pool continuously. When it is blocked or undersized for your pool’s volume, chemicals sit in concentrated pockets near the return jets while other areas of the pool receive little or no treated water. Algae grows in those untreated areas — typically the far corners of the pool floor and the areas behind steps or benches — and spreads from there.

A blocked filter also means your pump is working harder than it should, increasing electricity consumption and shortening the pump’s lifespan. In Uganda’s dusty environment, filter media becomes blocked significantly faster than in cleaner air conditions — particularly during the dry season when airborne dust is at its highest.

This is the most seasonally specific cause in Uganda — and the one that catches the most pool owners off guard. During Kampala’s rainy seasons, a single heavy storm can add hundreds of litres of unchlorinated water to your pool within hours. This dilutes your chlorine level directly. It also introduces organic matter — dust, leaves, soil runoff, and airborne contaminants — that consumes your chlorine rapidly. And it can drop your pH, which as discussed in Cause 2, makes whatever chlorine remains significantly less effective.

A pool owner who maintains correct chemistry through the dry season but does not adjust their routine during the rainy season will almost always experience green water outbreaks. The rainy season in Uganda is not a time to maintain — it is a time to increase frequency and dosing.

The most preventable cause of green pool water in Uganda — and the most common reason Pool Doctors Uganda is called for an emergency treatment. Pool maintenance only works when it is consistent. Skipping two weeks of chemical treatment does not mean the pool goes without chemicals for two weeks. It means the protection built up by the previous treatment is depleted, algae spores begin multiplying, and by the time treatment resumes, the pool has already turned or is on the verge of turning.

In Uganda’s warm temperatures, algae multiplication is significantly faster than in cooler climates. What would take three weeks to become visible green water in Europe can happen in five to seven days in Kampala during a warm period. Consistency is not optional. It is the entire maintenance strategy.