Pool Staining in Southwest Florida Is Usually Not Your Water

Pool staining in Southwest Florida usually comes from metal introduced at the pool rather than from the water used to fill it. The three sources that survive scrutiny are rusting fasteners in the screen enclosure directly above the waterline, metals left behind by earlier service products, and irrigation overspray carrying well water into a city-filled pool. University of Florida IFAS Extension puts the threshold at as little as 0.3 ppm of iron or manganese (UF/IFAS EDIS SS440)¹, which is a very small quantity of metal to have to account for.

Everyone says well water, so why do city-filled pools stain anyway?

That sentence is doing a specific job, and it is worth being precise about which one. "Staining means well water or bad chemistry" is a universal claim. It says every stained pool has one of two causes. A universal claim is broken by a single case that satisfies none of its causes and stains regardless, which is exactly what the forum report describes.

Take the falsification seriously and see what it removes. It removes fill water as the only mechanism, because the fill water in that case was municipal. It removes neglected chemistry as the only mechanism, because the chemistry was being held. It removes owner-added metal products, because none were used. Those three are the whole of the standard explanation.

What is left standing is metal arriving at the pool by a route other than the fill hose. There are three of those in this region, and they are the subject of the rest of this article: fasteners corroding in the screen enclosure directly above the water, metals introduced into the water by products added before you owned the pool, and irrigation overspray delivering water the pool was never filled with.

Now the honest part, because the strength of a claim is not the same as the strength of its evidence. That report is a working practitioner writing on a public forum. It has no sample, no controls, no published method and no way to check it. It is not a study, and this article does not treat it as one. It is one well-described counterexample.

A single counterexample is enough to falsify a claim stated as always, and it is not enough to tell you which cause is most common in Collier County. No source in this article establishes that ranking, and this article does not offer one. Anyone who publishes a percentage split of stain causes for this region is either citing something we could not find or making it up.

What do the Collier area utilities actually publish about the water?

The Collier County Water Sewer District states that hardness delivered in 2024 ranged from 42 to 94 mg/L, or 2.4 to 5.5 grains per gallon, averaging 67 mg/L (Collier County Water-Sewer District, 2024)². The City of Naples Utilities Division reports a 2024 range of 58 to 102 mg/L with an average of 67 mg/L, which it converts to an average of 3.91 grains per gallon (City of Naples Utilities Division, 2024)³. Two separate utilities, two separate documents, one number.

Collier also describes what it does to the water: it is treated with chloramines for disinfection and a corrosion inhibitor is added to prevent corrosion of pipes (Collier County Water-Sewer District, 2024)². Read that last part carefully. The inhibitor exists to protect the distribution system and the plumbing in your walls. It is a statement about pipe, not a statement about what the water does once it is sitting in a plaster shell in the sun.

What that water does to a fresh interior finish is a separate subject with its own answer, and it is covered in what Collier County tap water actually does to a plaster finish. This article is about metal, which is a different problem with different sources.

Here is the limit, and stating it is mandatory rather than optional. Neither Consumer Confidence Report publishes an iron figure. Iron is a secondary standard, and a Consumer Confidence Report is not required to report one. So this article does not say that your city water contains no iron, because no document we can cite says that.

That cuts in both directions, which is what makes it useful. The utilities' own reports cannot prove the fill water is carrying metal to your pool, and they cannot prove it is not. A document written for an entire service area cannot answer a question about one address. What answers it is a sample pulled from the hose bib you actually fill from, tested for metals. That test costs almost nothing and it removes a third of the uncertainty in one step.

Can rusting screen enclosure fasteners really stain a whole pool?

Start with the geometry, because it is the part that gets skipped. A screened cage is not a decoration sitting near the pool. It is a metal frame held together by fasteners, and a large share of those fasteners sit in a band directly over open water. Anything that corrodes on them has one place to go. Gravity does the rest, every time it rains and every time somebody hoses the deck down.

The atmosphere those fasteners live in is not a matter of opinion. NASA runs a beachside atmospheric corrosion test site in Florida, 150 feet from the mean high tide line, and reports chloride deposition there varying between 70 and 7,300 mg per square meter per day with a weighted mean of 983 (NASA Kennedy Space Center, 2023)⁴. For scale, the highest chloride deposition band in the ISO 9223 corrosivity standard covers 300 to 1,500 mg per square meter per day (NASA Kennedy Space Center, 2023)⁴. The site sits above the middle of that band on its weighted mean alone.

That site is the right comparison for a reason worth naming. It is Florida, it is coastal, it is instrumented continuously, and it has decades of record behind it. The alternative is pool industry copy asserting that salt air is hard on metal without ever measuring it. One of those two can be checked.

Now the finding that reverses what almost everyone assumes. Corrosion rates at that site are highest in the winter months, because lower temperatures let morning condensation sit on a surface far longer before it evaporates, winter winds and wave action put more salt onto the surface, and the dry season means less rain rinsing that salt back off (NASA Kennedy Space Center, 2023)⁴. Summer is not the season that eats your cage. Winter is.

It is also not a problem you buy your way out of with a better alloy. In a 20-year marine exposure study at the same program, the austenitic stainless alloys 304L, 316L and 317L all showed significant localized pitting corrosion across every sampled configuration, worst at the weld locations (NASA Kennedy Space Center, 2023)⁵. Better metal corrodes more slowly here. It does not sit still.

All of which explains why pools closest to the Gulf on Marco Island carry the sharpest version of this, and why a stain that concentrates in vertical runs under the cage line is telling you something a stain spread evenly across the floor is not.

How do metals from earlier service work and irrigation overspray get into a pool?

Metal does not evaporate. A copper based algaecide or a metal bearing treatment added to a pool two owners ago is still in that water in some form until it is removed deliberately or the water is replaced. Nobody hands over a log of what went in. The new owner, and often the new service company, inherit a chemistry they did not create and cannot reconstruct.

This is the quietest of the three causes because it leaves no visible hardware to inspect. There is no rust run to point at and no sprinkler head to watch. It shows up as staining that appears reasonably evenly rather than in runs, on a pool whose hose bib tests clean, in an enclosure with no visible corrosion. That combination is a result, not a dead end.

The irrigation route is the one most owners have never considered, and it runs on a published timetable. Collier County limits irrigation to three days a week by address: even addresses on Tuesday, Thursday and Sunday, odd addresses on Monday, Wednesday and Saturday, never between 10 a.m. and 4 p.m., and never on Friday (Collier County Public Utilities, 2026)⁷. If a head throws far enough to reach the water or the shell, you know exactly which mornings it lands.

The reason that matters is that irrigation water is frequently not the water you fill with. A great many properties in unincorporated Collier irrigate from a private well while filling the pool from the utility. Measured iron in Collier County groundwater monitoring wells runs a median of 139 µg/L across samples collected between 2015 and 2025 (USGS and US EPA Water Quality Portal, 2025)⁶, against a staining threshold that UF/IFAS puts at as little as 0.3 ppm, which is 300 µg/L (UF/IFAS EDIS SS440)¹.

Those are ambient monitoring wells rather than homeowner supply wells, so they describe the groundwater in the county and not the water at any particular house. The point is not the number. The point is that a pool can be filled with one water and sprayed weekly with a completely different one, and every conversation about the fill water misses that entirely.

A pool actually filled from a well is a different subject with its own numbers, and it is worked through in what well water does on the other side of the county. The case here is narrower and stranger: a city-filled pool receiving well water anyway, a few mornings a week, through a sprinkler nobody thinks of as plumbing.

How do you separate the three causes without guessing?

Those three observations, in that order, cost nothing and take one morning. They do not diagnose the pool. What they do is turn one open question into three closed ones, and closed questions are the only kind anybody can act on.

Take the water sample from the bib you genuinely fill from, not a different tap on a different side of the house, and ask for metals rather than a general test. If it comes back with measurable iron or copper, the fill water is contributing and that is worth knowing before anything else is decided. If it comes back clean, you have removed the explanation that everybody reaches for first, and the remaining causes get the attention they should have had.

The cage inspection is visual and it is about direction. Staining that starts above the waterline and runs downward is reporting something above the water. Staining that is heaviest directly under the screen channel or under a fastener line has a source you can point at. Even staining across the floor of the pool, with no vertical runs and nothing corroding overhead, is pointing somewhere else.

The irrigation check needs the system running. Watch where each head actually throws rather than where it was aimed when it was installed, because heads drift and landscaping grows. If one reaches the water or wets the shell, note it, and note which days it runs under the county schedule⁷.

Then the honest ceiling, which most pages on this subject never reach. Some stains are no longer sitting on the finish. Metal that has been taken up into plaster does not come back out with a chemical, and the test spot tells you which situation you are in: treat a small area, record what it looked like first, and wait. A stain that lifts and stays gone was on the surface. A stain that returns within weeks is reporting either a source that is still delivering metal or a finish that has already absorbed it.

That is where when a stain has gone into the finish rather than sitting on it stops being a treatment question and becomes a resurfacing one, and where a cause that is still active has to be stopped first, because a new finish under a live source stains again. If the source turns out to be equipment, hardware or a leak feeding the problem, pool repair in Naples is the part that comes before the finish.

The bridge, stated plainly: if the test spot comes back clean and stays clean, you do not need us yet, and the money is better spent stopping whatever put the metal there. If it comes back the same color it started, a resurface is the honest answer rather than the fourth bottle.