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How to fix a green pool

A green pool is not "short of algaecide". Something let the chlorine fall. Clear the pool, then find and fix the cause, or it will be green again in two weeks.

Why pools go green

Algae spores blow into every pool every day. They grow when free chlorine stays too low for long enough, especially in warm, sunny weather. The usual causes are:

• missed doses, a holiday, or a broken chlorinator;
• high pH, which makes chlorine weak;
• high stabiliser, which locks chlorine up;
• a pump that is not running long enough, a power cut, or a broken time clock;
• a dirty filter or worn-out sand;
• storms, which dilute the chlorine and wash in dust and leaves;
• heat.

Recovering a green pool, step by step

1. Safety first. Nobody swims while you cannot see the bottom. A person, a pet or a broken step could be hidden.
2. Remove the debris with a leaf rake before you add anything. Leaves use up chlorine.
3. Get the water moving. Empty the baskets, backwash the filter, and run the pump 24 hours a day until the pool is clear. Take the automatic cleaner out.
4. Test and balance the pH to about 7.2 to 7.4.
5. Shock hard. A green pool needs much more chlorine than a routine shock. Follow the product label's directions for algae or green pools, and keep the chlorine high, testing and re-dosing, until the water stops being green.
6. Brush every wall and the floor, twice a day if you can.
7. Filter and backwash. Dead algae clogs the filter fast, so backwash whenever the pressure rises.
8. Clear the cloud. When the water turns grey-blue and cloudy, the algae is dead. Let the filter clear it, with a clarifier to help, or use a flocculant and vacuum the sludge to waste.
9. Rebalance everything and bring the chlorine back to normal.
10. Find and fix the cause.

Kinds of algae

• Green algae: the most common. It clears with chlorine, brushing and filtering.
• Mustard algae: yellow-brown dusty patches in shady spots. It resists normal chlorine and comes back from brushes, cleaners and swimwear unless you clean those too.
• Black algae: dark spots with roots in the plaster. It needs scrubbing, chlorine applied right on the spots, and patience.

About algaecides

An algaecide works alongside chlorine, never in its place. Copper-based products can stain the pool blue-green or black, and some foam if you overdose. Follow the label.

Clarifier or flocculant?

• Clarifier: clumps fine particles so that the filter can catch them. Keep the pump running. It takes a day or two and wastes little water.
• Flocculant: drops the particles to the floor as sludge. Run the pump briefly to mix, then switch it off for 8 to 24 hours, and vacuum slowly to waste. It works overnight but uses several centimetres of water. Do not use it with a cartridge filter.

Other causes of cloudy water

• Poor filtration: run the pump for longer, backwash, and check the sand.
• High pH, alkalinity or hardness: balance the water.
• Dust after a storm: filter for longer.
• Many swimmers after a party: shock and filter.

Stains

• Brown or green patches shaped like leaves: organic. Brush and shock.
• Rust or orange-red: iron, from borehole water or metal objects on the floor.
• Blue-green or black: copper, from copper algaecides or corroding fittings.
• White crust at the waterline: scale, from hard water with a high pH.

A simple test: rub a vitamin C tablet on a small stain under water. If the stain lightens quickly, it is probably metal.

Common complaints explained

• A strong "chlorine" smell means too little free chlorine, not too much. Shock the pool.
• Stinging eyes usually mean that the pH is out of range.
• Blonde hair turning green is caused by copper in the water, not chlorine.

A neglected pool breeds mosquitoes

Mosquitoes can breed in a neglected pool within about a week. Get the water circulating and chlorinated, or cover the pool.

This guide is based on Chapter 9 of Swimming Pool Maintenance in South Africa.