Almost always - either the free chlorine is genuinely low, or pH is high enough that the chlorine present is not working.

Adequately sanitised, balanced water does not grow algae. A bloom is evidence that something in the sanitising regime has failed.

The obvious cause is a low residual, from under-dosing, heavy demand, or stabiliser loss on an outdoor pool after rain.

The less obvious cause is high pH, which reduces the active fraction of the chlorine present. The test looks fine and the water goes green anyway.

High cyanuric acid produces the same effect - chlorine measured but over-stabilised and sluggish.

Poor circulation creates local failures: a shaded corner, behind a ladder, under steps or in a dead spot where treated water never reaches.

Treat the bloom, then find which of these caused it, or it recurs.

Polymeric quats for routine prevention and green algae, copper for yellow and black algae, with brushing essential for anything clinging to a surface.

Green algae is free-floating and responds to shock chlorination plus a routine algaecide. It clears fastest.

Yellow or mustard algae clings to shaded walls and tolerates normal chlorine levels. It needs brushing, elevated chlorine and usually a copper-based product, plus attention to anything that entered the pool from an infected source.

Black algae roots into porous surfaces and forms a protective layer. Brushing hard enough to break that layer is not optional, and treatment usually needs to be repeated over several days.

Quaternary ammonium products are the cheapest but foam when over-dosed, which is unpleasant and hard to remove.

Whatever is used, filtration must run continuously and the filter needs cleaning afterwards - dead algae is a substantial load.

It can, particularly if the water is unbalanced, if the dose is excessive, or if the pool surface is plaster - and copper staining is difficult to remove.

Copper stays in the water until removed. If pH rises or the water chemistry shifts, dissolved copper can precipitate onto surfaces as blue-green or black staining.

Plaster and tiled pools are most affected; vinyl is more tolerant but not immune.

Copper also reacts with chlorine and with hair, producing the green tint that swimmers occasionally report - it is copper, not chlorine.

The controls are dosing accurately, keeping pH and alkalinity in range, and not repeatedly topping up copper products through a season.

Chelated copper formulations reduce but do not eliminate the risk.

Where staining matters, use a non-metallic algaecide for routine prevention and reserve copper for genuine problem algae.

Yes - a low maintenance dose costs far less than clearing a bloom, and blooms cost pool closure as well as chemicals.

Clearing an established bloom requires shock chlorination, repeated algaecide, extended filtration, brushing, filter cleaning and usually a period with the pool out of use.

A preventive dose added routinely through the season keeps the algae population from establishing at all.

It is particularly worthwhile on outdoor pools, in warm weather, on pools surrounded by planting, and where the chlorine residual is occasionally allowed to drift.

It does not substitute for sanitiser. A pool relying on algaecide with poor chlorine control has a hygiene problem regardless of how clear the water looks.

Dose after backwashing and at the start of the season, and follow the product's maintenance rate rather than dosing heavily on an irregular basis.

Because that place has poor circulation, shade, or a surface the algae has rooted into - the chemistry is reaching the rest of the pool and not that spot.

Dead spots behind ladders, under steps, in corners and around fittings receive very little treated water. Sanitiser concentration there is lower than the test at the skimmer suggests.

Shade reduces ultraviolet and keeps the surface cooler and damper, which suits several algae types.

Porous or damaged plaster gives black algae somewhere to root, and cleaning the surface does not remove what is below it.

The fixes are physical as much as chemical: adjust return jet direction to circulate the dead spot, brush the area routinely, and repair damaged surfaces.

Recurring algae in one place is a maintenance problem, and dosing the whole pool more heavily is an expensive way not to solve it.

Phosphates feed algae, so removing them reduces the nutrient supply - but they are not the cause, and a well-sanitised pool with phosphates present does not bloom.

Phosphates enter from fill water, fertiliser run-off, leaves, cosmetics and some chemical products. They are a nutrient that algae uses.

Phosphate removers precipitate them for filtration. In pools with a persistent problem and a heavy nutrient source, they genuinely help.

They are, however, sometimes sold as a solution to algae in pools whose actual problem is inadequate chlorine or high pH. Removing the food does not fix the sanitiser.

The sensible order is: correct the sanitiser and the balance, address circulation and brushing, and consider phosphate removal where blooms persist despite all of that.

Filtration load rises after phosphate treatment, so expect to clean the filter.

It depends on the product - some allow swimming almost immediately, others require a wait, and the label is the authority.

Routine maintenance doses of polymeric algaecides typically allow bathing after a short circulation period.

Higher treatment doses, and copper-based products in particular, usually specify a longer interval.

Where an algaecide has been combined with shock chlorination - which is common when clearing a bloom - the chlorine level governs, and bathing waits until free chlorine has fallen back into the operating range.

Public pools operating under a standard will have their own rule on reopening after chemical treatment, usually requiring a verified test.

Always circulate for the stated period before reopening, so the dose is distributed rather than concentrated where it was added.