Why Is My Pool Cloudy?

Cause 1: the filter, which is most of them

Start here, because it is free to check and it is the answer more often than everything else combined. A filter that is loaded past capacity passes water around the media rather than through it. The pump still runs, the returns still flow, and nothing is being filtered.

• **Check the pressure gauge.** Around 8 to 10 psi above the clean-filter reading means it needs attention. If you never recorded the clean reading, do it after the next clean.
• **Check the run time.** Most residential pools need eight to twelve hours of circulation a day in season. A pool cloudy on a six-hour timer often needs nothing but more hours.
• **Check the media age.** Sand lasts three to five years, cartridges one to three seasons, DE grids several years. All three fail gradually and invisibly - see pool filter cartridges for the consumable most people run too long.
• **Check for air.** Bubbles at the returns mean a suction-side leak, and a pump drawing air is not moving the water it appears to be.

Cause 2: pH and alkalinity

High pH causes cloudiness directly, by pushing dissolved calcium out of solution as fine particulate. It also blunts your chlorine at the same time, so a pool with pH at 8.2 is both hazy and under-sanitized while its chlorine reading looks acceptable.

HTH's own pH Down page states the target as 7.2 to 7.6, and its Alkalinity Up page gives 80 to 120 ppm for total alkalinity. Correct alkalinity first: it is the buffer that stops pH bouncing, so fixing pH on a pool with low alkalinity means fixing it again next week.

Cause 3: dead algae

If the pool was green in the last week, this is your answer. Gray-cloudy water after a shock is dead algae in suspension, and it is what success looks like. See cloudy pool water after shock.

The tell is color: dead algae is gray or gray-green rather than milky white, and it settles into a fine layer on the floor overnight that puffs up when disturbed.

Cause 4: swimmers, oils and sunscreen

Every swimmer brings sunscreen, body oil, lotion, hair product and sweat. None of it filters out easily, and all of it consumes chlorine. A pool that is clear on Friday and cloudy on Sunday after a party has this cause, not a chemistry one.

The distinguishing feature is timing - it correlates with use rather than with weather - and often a persistent foam. See cloudy from sunscreen and swimmers.

Cause 5: particulate below the filter's rating

This is the case a clarifier is genuinely for, and it is the last one to conclude rather than the first. Sand filters catch down to roughly 20 to 40 microns, cartridges to about 10 to 20, DE to around 5. Anything finer passes through as many times as you circulate it.

Fine silt after a storm, pollen in spring and very fine dead algae all sit in this band. A clarifier clumps them until they cross the filter's threshold. See best pool clarifier.

Cause 6: calcium hardness

Water that is saturated with calcium deposits the excess as fine crystals, which is a persistent milky haze plus scale at the waterline. It is most common where fill water is hard, or after many seasons of cal-hypo shock.

There is no chemical that removes calcium hardness. The fix is dilution - a partial drain and refill with softer water - which is genuinely the answer and genuinely unwelcome. If your fill water is the source, that is also an argument for switching from cal-hypo to liquid chlorine.

Working through it in order

1. Read the filter pressure gauge and the run time. Clean or backwash if either is off.
2. Test pH and total alkalinity. Correct alkalinity first, then pH.
3. Ask whether the pool was green or shocked in the past week.
4. Ask whether the cloudiness followed heavy use rather than weather.
5. Test calcium hardness if the fill water is hard or there is scale at the waterline.
6. Only if all five come back clear, buy a clarifier - because now you know it is fine particulate.