Why is my pool cloudy? Causes, fixes, and what a service tech actually does

The 5 causes that produce almost every cloudy pool

• Chemistry imbalance (low chlorine, low pH, low alkalinity, or any of these out of balance with each other)
• Filter problem (dirty, undersized, clogged, or broken)
• High bather load (recent pool party, swim lessons, multiple kids)
• Dead algae (recent SLAM or shock, suspended particles clearing)
• High calcium hardness (above 400 ppm, calcium precipitating out)

The diagnosis order

The test order matters because each result either confirms or rules out a cause. The fastest diagnosis runs tests in this sequence:

Step 1: free chlorine. If FC is below 1 ppm, the pool is undersanitized and the cloudiness is the early stage of an algae bloom about to turn the pool green. Shock and treat as a green pool recovery. Done.

Step 2: pH. If pH is above 7.8, calcium starts precipitating out of the water and chlorine is half as effective. The water looks hazy because of suspended calcium crystals. Drop pH with muriatic acid first, then re-evaluate.

Step 3: total alkalinity. If TA is above 120 ppm, calcium precipitation accelerates. Lower TA with controlled acid additions and aeration.

Step 4: calcium hardness. If CH is above 400 ppm (or 350 ppm in hard-water markets), the pool is over-saturated and you need partial drain and refill. There is no chemistry shortcut.

Step 5: filter pressure. If pressure is 8 to 10 psi above clean baseline, the filter is restricted and not catching fine particles. Backwash (sand) or clean (cartridge or DE) and re-evaluate after 24 hours of filtration.

Cause 1: low free chlorine

A pool with free chlorine below 1 ppm has lost its sanitizing buffer. Bacteria, algae, and organic particles accumulate faster than the filter can clear them. The first visible sign is cloudiness; if left another 24 to 48 hours, the pool turns green.

Fix: shock with 12.5% liquid chlorine at the SLAM target for the pool's CYA level (1-3 gallons for a typical residential pool). Run filter 24/7. Brush walls and floor. Retest in 12 hours. Most light cloudy pools from low chlorine clear within 24 to 36 hours.

Cause 2: dirty or undersized filter

A filter that runs at 8 to 10 psi above its clean baseline is not catching fine particles. The water can have perfect chemistry and still look cloudy because the suspended dust, dead algae, and bather body oils are recirculating instead of getting trapped.

Fix: backwash a sand filter, hose a cartridge, or fully break down and recharge a DE filter. After cleaning, run the filter 24 hours straight. If the pool clears, the filter was the cause. If it does not, the filter may be undersized for the pool's gallons or the pump is underpowered. Both are equipment problems that require replacement, not chemistry.

“A clean filter and balanced chemistry will fix 80% of cloudy pools without any other intervention.”

Cause 3: high bather load

Pools recover slowly from heavy use. A pool that hosts 8 kids and 4 adults for a 4-hour party absorbs roughly 12 oz of sweat, sunscreen, body oils, and other organics. The chlorine demand spike is dramatic; the filter is hit with a flood of particles all at once.

Fix: shock the pool the night of (or morning after) heavy use at 2x normal dose. Run the filter 24 hours. Add 8 oz of pool clarifier to bind fine particles into larger clumps the filter can catch. Most party-cloudy pools clear within 24 to 36 hours.

Cause 4: dead algae from a recent recovery

A pool that went green and got SLAMed will look cloudy for 2 to 5 days after the algae dies. The dead algae particles are too small for many filters to catch immediately, and they recirculate through the water column.

Fix: keep the filter running 24/7 for the full recovery window. Vacuum to waste if particles settle to the bottom overnight. Add a clarifier to clump small particles. Patience is the main treatment; this clears on its own once filtration has time to work.

Cause 5: high calcium hardness

In hard-water markets (Phoenix, Las Vegas, parts of Texas), calcium hardness creeps up over years as evaporation concentrates dissolved minerals. Above roughly 400 ppm (or 350 in extreme markets), calcium starts precipitating out of solution as a fine white haze. No amount of chemistry will fix this; the water is simply over-saturated.

Fix: partial drain and refill. 50% drain typically halves the calcium hardness. After refill, restart chemistry from scratch (CYA, alkalinity, pH, then chlorine). This is the only durable fix. Some operators sell a "scale inhibitor" product to mask the symptom; it works for 3 to 6 weeks and then the haze comes back. Drain is the real answer.

What does not work

• Pouring more chlorine into an already-cloudy pool with high pH. Chlorine is half-effective above pH 7.8; you are wasting it
• Skipping the filter clean. Even perfect chemistry will not clear if the filter is restricted
• Adding a clarifier without addressing the underlying cause. Buys you 24 hours then the haze returns
• Telling the customer "give it a week." Customers expect daily visible progress on a service call. Communicate the diagnosis and the timeline in writing the same day

What to charge for a cloudy pool diagnosis

A cloudy pool service call is typically priced as a diagnostic visit ($75 to $150) plus chemicals at cost plus 20 to 30% markup. If a drain and refill is required, that is a separate $400 to $800 plus water cost. Customers who get an itemized written estimate before work starts are the customers who pay without disputes. Skip the verbal "should be a few hundred bucks" framing; it always blows up.