Why Is My Pool Green? Causes and Fixes for St. Petersburg Pools

Reviewed by Dylan Pawley, Owner, Brightwater Pool Co · Serving St. Petersburg, St. Pete Beach, and Pinellas County · Last updated August 2026

1. Chlorine dropped too low

Algae spores are already in your water. They multiply fast once free chlorine falls below roughly 1 ppm, which is also the floor the CDC sets for a minimally sanitized residential pool. A missed week of dosing, a hot stretch of Florida sun that burns off unstabilized chlorine faster than usual, or a pool left untreated during a vacation is often all it takes.

The fix is straightforward once you catch it early: test free chlorine and pH before adding anything, then shock to bring free chlorine well above the algae threshold and keep the filter running around the clock until the water clears.

2. pH is too high for chlorine to work

This is the cause behind most "I shocked it and nothing happened" cases. At a pH of 8.0, chlorine operates at a fraction of its normal disinfecting strength, even though a test strip still shows chlorine in the water. The CDC's operating range for residential pools is pH 7.0 to 7.8, with most pool professionals targeting 7.2 to 7.6 as the practical sweet spot.

How to check it

Test pH and cyanuric acid (CYA) before assuming a chlorine reading is telling you the whole story. If pH is above 7.6 or CYA is above 75 ppm, chlorine is present but largely inactive, a condition sometimes called chlorine lock. Correct pH first, then re-shock.

3. Heavy rain diluted and contaminated the water

St. Petersburg's summer storms are a common trigger. A single downpour dilutes your chlorine concentration, washes phosphates and organic debris off the deck and surrounding landscaping into the pool, and can shift pH, all in one event. It is common for a pool that was clear on Friday to be visibly green by Sunday morning after a weekend storm.

After any significant rain, test chemistry before you assume the pool is fine. A pool that looks clear immediately after a storm can still turn a day or two later as chlorine finishes dropping and phosphates start feeding algae.

4. Phosphates are feeding the algae

Phosphates come from fertilizer runoff, decaying leaves, pollen, and even some pool chemicals, and they act as fuel for algae the same way fertilizer feeds a lawn. Chlorine kills algae, but it does not remove phosphates, and dead algae actually release more phosphates back into the water. That is why a pool can bloom green again within days of a shock treatment that looked successful.

When to suspect this

If a pool keeps returning to green despite consistent chlorine dosing and a corrected pH, phosphates are the most likely remaining cause. A dedicated phosphate test kit (phosphates are measured in parts per billion, not the parts per million used for chlorine) is the only reliable way to confirm it. Once phosphates are treated, chlorine typically has an easier time holding a clean pool.

5. Poor circulation or filtration

Even with correct chemistry, algae can take hold in a pool with a weak pump, a clogged filter, or a pump that simply is not running long enough to turn over the full volume of water each day. Dead spots near steps, ladders, and shallow corners are the first places algae shows up when circulation is inadequate.

If green patches appear in specific corners while the rest of the pool stays relatively clear, filtration and circulation are the more likely cause than chemistry alone.

The right order to treat a green pool

Step	What to do	Why it matters
1. Test first	Check free chlorine, pH, and CYA before adding anything	Adding chlorine to water with high pH or high CYA wastes product and time
2. Correct pH	Bring pH into the 7.2 to 7.6 range	Chlorine cannot do its job outside this range, regardless of dose
3. Brush and clean	Brush walls, steps, and corners; clean skimmer baskets and filter	Breaks up algae colonies so chlorine can reach them
4. Shock	Add enough chlorine shock to overwhelm the chlorine demand	A single normal dose is rarely enough for an active bloom
5. Run the filter continuously	Keep the pump and filter running 24/7 until the water clears	Filtration is what actually removes the dead algae from suspension
6. Address phosphates	Treat with a phosphate remover if the pool keeps returning to green	Removes the algae's food source so it does not come right back

When to call a professional instead

A mild, early-stage green tint is often manageable with a single shock and a day or two of continuous filtration. A pool that has been neglected for weeks, is closer to black or swamp-colored, or keeps returning to green after repeated shock treatments usually needs a professional Green-to-Clean service. At that point the water chemistry is often compounding multiple problems at once, and dosing more chemicals without addressing the underlying cause tends to waste money without fixing anything.

Frequently asked questions