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A pool does not turn green overnight, but it can get there surprisingly fast when heat, strong sun, weak chlorine and a stopped filter come together. In a Riyadh summer, a few days of neglect can change the water colour, especially after a sandstorm or while the family is away on holiday. Algae is not just unsightly. It shows that the water has lost its balance, and if left for a while it starts to affect tiles, grout, the filter and swimmer safety. This article explains how algae forms, the types you may see, the damage it causes over time and how to treat it properly instead of throwing in chlorine and hoping.
How algae forms
Algae spores arrive constantly in air, dust, rain and even on swimwear. Their presence is normal. Growth is the problem, and it happens when conditions allow:
- Weak or spent chlorine: chlorine kills spores before they multiply. When it drops, growth starts quickly.
- Unbalanced pH: high pH reduces chlorine's effectiveness even when the amount seems enough. Aim for about 7.2 to 7.6.
- Warm water and strong light speed up growth, which is why problems peak in summer.
- Poor circulation: corners, steps and behind ladders hold still water where algae starts.
- Nutrients: dust, leaves and phosphates from fertilisers and cleaning products feed algae.
Types of algae and how to recognise them
Green algae
The most common. It starts as a light green haze or slimy green patches on walls and floor. It spreads fast but is the easiest to treat.
Yellow or mustard algae
Appears as yellow or brown dust on shaded walls and is often mistaken for sand. It lifts when brushed, then settles again, and is more stubborn than green algae.
Black algae
Small black or dark blue spots, hard to the touch, rooted into grout and rough surfaces. It is the hardest type, and incomplete treatment lets it return in the same spot.
Pink slime
Actually a type of bacteria, seen as pink streaks in damp places like skimmer edges and corners. It needs focused cleaning and treatment.
Long-term damage
- Grout and tile damage: black algae penetrates grout and breaks it down, leaving gaps that let water reach the waterproofing.
- Permanent stains on tiles, paint and plaster that remain even after the algae is dead.
- Filter and pump strain: dead algae and slime clog the filter, pressure rises, the pump works harder and backwashing becomes frequent.
- Higher chemical use: an algae-laden pool consumes far more chlorine, and late treatment takes more product than prevention.
- Shelter for bacteria: algae protects some bacteria from chlorine, so water can be unsafe even when it looks better.
- Slip risk from slime on steps and floor, especially for children.
- Hidden floor: cloudy water hides the bottom, which is a real safety hazard.
Step-by-step treatment
- Remove debris such as leaves, insects and settled dust with a net.
- Test the water for pH, chlorine and alkalinity, and correct pH first, since chlorine works poorly in unbalanced water.
- Brush every surface with a brush suited to the finish. Brushing breaks the algae's protective layer. Black algae needs firm, spot-by-spot brushing.
- Shock treatment: raise chlorine temporarily according to water volume, severity and product instructions, preferably in the evening so the sun does not burn it off.
- Run the filter continuously until the water clears, watching filter pressure and backwashing when it rises.
- Vacuum the floor once dead algae settles, ideally to waste if there is a lot, so it does not return to the filter.
- Retest: nobody swims until chlorine is back to normal and the water is clear.
In severe cases, with dark green water and no visible floor, or widespread black algae, professional cleaning may be needed and sometimes draining and washing the surfaces. That decision needs assessment because draining without planning carries its own risks.
Keeping algae away
- Test chlorine and pH regularly, more often in summer and after heavy use.
- Run the filter long enough each day and aim the return jets to move water in the corners.
- Brush walls and steps regularly even when the water looks clear.
- Empty the skimmer and pump baskets, and backwash based on the pressure gauge.
- After sandstorms, clean the surface and test the water straight away.
- Before travelling, arrange for a technician to look after the pool or at least a reliable running and sanitising routine.
For more on the chemistry, read about the damage caused by neglected water treatment.
Quick summary
- Green water signals a problem with chlorine, pH or circulation.
- Identify the algae type first. Black algae needs more effort.
- Fix pH, brush, shock, then filter and vacuum.
- Never swim in water where you cannot see the floor.
- Regular prevention is far cheaper and easier than late treatment.
Frequently asked questions
Can I swim in a pool with light algae?
We do not recommend it. Algae means sanitising is weak, it can shelter harmful bacteria and it makes steps slippery. Treat the water first and confirm it is clear with normal chlorine.
Why does algae come back after treatment?
Usually because pH was not corrected, surfaces were not brushed enough or the filter was not cleaned afterwards. Black algae returns if its roots stay in the grout.
Do I need to drain the pool to get rid of algae?
In most cases no. Chemical treatment with brushing and filtration is enough. Draining is for very severe cases after assessment, because it carries risks for the shell and finish.
Can algaecide replace chlorine?
No. Algaecide helps with prevention, but balanced chlorine and good filtration remain the foundation.
