Published November 13, 2026 by A Clean Pool USA
The test strip says the pool is fine. Chlorine is in range, pH is in range, and a few days after every cleaning there is a green film on the steps again. We see it on pools across Central Florida. The chlorine reading usually is not wrong. It just leaves things out. Three things are usually behind it: stabilizer that has crept up over the months, and the two nutrients algae feed on, phosphate and nitrate.
This post is the short version. For the studies, the regulations and the chemistry behind each point, see our research review on phosphates, nitrates and pool algae.
Not All of the Chlorine You Measure Is Working
Stabilizer (cyanuric acid, often written CYA) protects chlorine from the Florida sun. It does that by holding on to the chlorine. Held chlorine is protected, but it kills slowly. In a stabilized pool, more than 99 percent of the chlorine is in that held form. Your test kit counts all of it.
So 3 ppm of chlorine at 30 ppm stabilizer and 3 ppm at 100 ppm stabilizer are very different pools, even though both "test fine." What counts is chlorine compared with stabilizer. The CDC's model code for public pools treats stabilizer at more than 45 times the chlorine level as an imminent health hazard, and a 2019 study recommended keeping it at no more than 20 times. A pool at 100 ppm stabilizer and 3 ppm chlorine sits at about 33 times.
Why Stabilizer Keeps Climbing in Florida Pools
Chlorine tablets (trichlor) are more than half cyanuric acid by weight. Every 1 ppm of chlorine a tablet feeder adds also adds about 0.6 ppm of stabilizer. Stabilizer does not evaporate and does not get used up, and chlorine cannot burn it off in any practical amount of time. The only way it leaves is with water: splash-out, backwashing, rain overflow, or draining. A pool that runs on tablets all year keeps adding stabilizer, and nothing in normal pool chemistry takes it back out.
Where Phosphates and Nitrates Come In
Algae are plants, and like any plant they need phosphorus and nitrogen to grow. Lake scientists showed this decades ago. In a well-known Canadian experiment, researchers divided a lake into two basins and fed both nitrogen and carbon, but gave phosphorus to only one. Only the basin that got phosphorus bloomed.
In a pool where chlorine is strong all the time, nutrients matter much less, because algae die before they can use them. But chlorine does not stay strong every hour of the week. A hot afternoon, a thunderstorm, a pool party or a tablet feeder that runs dry can all drop it for a while. When that happens, nutrients decide how fast algae comes back. With low nutrients, a dip in chlorine is a small setback. With high nutrients, a dip on Thursday can be a green pool by Monday.
One more wrinkle: when phosphorus is plentiful, algae store extra inside their cells and draw on it later. Knocking phosphate down halfway, once, does less than keeping it low.
Where They Come From in Central Florida
- Your tap water. Several local utilities add phosphate to drinking water to protect pipes from corrosion. Their own water reports name it: Sanford (polyphosphate), Altamonte Springs (orthophosphate), Casselberry (orthopolyphosphate) and Deltona (poly-orthophosphate). In Toho Water Authority's Bellalago system, the state requires at least 0.7 mg/L of orthophosphate in the water. Every top-off adds a little. Our guide to Central Florida pool water by utility has the details for 21 utilities.
- Lawn fertilizer. Runoff and overspray carry nitrogen and phosphorus into the pool. Maitland bans lawn fertilizer containing nitrogen or phosphorus from June 1 to September 30 to protect its lakes, and the same nutrients reach pools.
- Leaves, pollen, dirt and swimmers. Organic debris breaks down into nutrients, and swimmers bring in nitrogen compounds.
- Some pool products. Some stain and scale preventers contain phosphonates, and chlorine can break phosphonates down into phosphate. Some algaecides and cleaners contain nitrogen compounds. Read the label.
The Honest Debate
Some pool experts say phosphate removers are a waste of money. Keep chlorine high enough for your stabilizer level, they say, and algae cannot grow no matter how much phosphate is in the water. On the chemistry, they are right. Even a company that sells phosphate remover says removers are "not required" and that chlorine at the right level for the stabilizer "should be able to stay ahead of algae." The same company adds that holding that level "can be very difficult and impractical to do, especially if your pool is overstabilized." That matches what we see.
Holding chlorine at the right level for your stabilizer every hour between weekly visits, in July, while tablets keep pushing stabilizer up, is hard. Lowering stabilizer and nutrients gives chlorine a wider margin, so a bad afternoon stays a bad afternoon and does not become a green pool. And a phosphate remover will not clear a pool that is already green. That takes chlorine, brushing and filtering.
There is not much controlled research on pools specifically. Most of the nutrient science comes from lakes. Our research review separates what has been measured from what is borrowed from lake science and what comes from experience.
How to Fix It
1. Test what usually goes untested
Chlorine and pH are only the start. Test stabilizer, phosphate and nitrate too, because those explain the pools that "test fine" and still grow algae.
2. Bring stabilizer down
The only practical way to lower stabilizer is to replace some of the water. The CDC's model code for public pools names draining and refilling as the way to bring stabilizer down when a pool has to be disinfected after a contamination incident. A partial drain is usually enough, and it is a service we do for our customers. One caution for Florida: our water table is high, and groundwater can push up on an empty or nearly empty pool shell. Do not drain a pool all the way without someone who knows how to check for that.
3. Keep chlorine in step with stabilizer
Once stabilizer is back in a sensible range, chlorine has to be held in proportion to it. The higher the stabilizer, the higher the chlorine needs to be.
4. Remove phosphate
Many phosphate removers use lanthanum (the label lists the active ingredient), which binds phosphate into a solid that does not dissolve. The filter catches it. The water may turn cloudy for a day while that happens, and the filter should be cleaned afterward.
5. Dilute nitrate
No pool chemical removes nitrate, and chlorine cannot break it down. The only fixes are replacing water or running the pool through reverse osmosis. When nitrate is high, a partial drain and refill is not wasted money. It is the treatment, and we can do it for you.
6. Cut off the sources
Keep fertilizer granules and grass clippings away from the pool, keep the deck clear, check the labels on stain and scale products, and keep the skimmer and pump basket empty so debris is not breaking down in the water.
Let Us Handle It for You
At A Clean Pool USA, we service pools every week across Central Florida, including Winter Garden, Windermere, Ocoee, Altamonte Springs, Casselberry, Sanford and Lake Mary. If your pool keeps turning green even though the chlorine reads fine, give us a call. We test stabilizer, phosphate and nitrate, and we handle partial drains and refills when the water needs replacing.
All chemicals are included with our weekly service plans, and your first month is free.