Pool Water Problems: Symptom to Fix

Something is wrong with your water and you don't know the word for it yet. That's where most pool problems actually start, and it's the part almost every pool article skips. They assume you already know you have mustard algae, or scale, or chloramines, and they begin from there. If you knew that, you wouldn't be looking.

So this page starts from what you can see standing on the deck. Find the thing your pool is doing, read enough to know what you're dealing with, run the one test that confirms it, and then go to the page that fixes it.

Almost none of this requires a diagnosis you can't make yourself. What it requires is knowing which test separates two things that look identical, and that's the part nobody writes down.

Find Your Symptom

If you only read one thing on this page, read that table. Everything below is the same information with enough detail to act on.

The Water Is Green

Green runs from a faint tint you notice against the steps all the way to water you can't see your own feet in. It's the most common pool problem there is and the easiest to identify, because green algae floats. That's why the water clouds as it gets worse, and it's what separates this from the algae further down the page.

The useful measure of how bad it is isn't the color, it's what you can still see. Look for the main drain.

1. You can make out the main drain. You're in decent shape.
2. You can see the shallow end floor but not the drain. You're in the middle.
3. You can't see the floor anywhere. The worst rung, and the one where draining part of the pool sometimes beats treating it.

Check free chlorine, and check your stabilizer at the same time. Chlorine on its own doesn't tell you much here, because how much a pool needs depends entirely on how much cyanuric acid is holding it — which is why dosing advice given in pounds per ten thousand gallons fails so many people.

If your chlorine genuinely is where it should be for your stabilizer and the pool is still green, or it greens up again within days of you clearing it, the next thing to look at is phosphates.

It helps to know what you're fighting. Green and mustard algae are plants in every way that matters to you — they photosynthesize, they grow, and they feed. Phosphate is fertilizer. Chlorine is still the thing that kills them, but water loaded with phosphates grows algae faster than an ordinary chlorine level can keep ahead of, which is how you end up dosing correctly and losing anyway. Black algae is the odd one out here, because it's a cyanobacteria rather than a plant, and that's part of why it behaves so differently from everything else on this page.

There's one case where green isn't algae at all, and the timing is what gives it away. If the water turned green within hours of you shocking it, and it stayed clear instead of clouding, you're probably looking at metals. Copper in particular will turn a pool green the moment you oxidize it, and no amount of chlorine will touch that.

The Water Is Cloudy or Milky

Cloudy water has several unrelated causes, and one branch decides most of them. It isn't a chemical question. It's whether your chlorine is holding.

Dose the pool, read your free chlorine, and read it again the next morning.

• The level held. The cloudiness is particulate — dead material, undissolved shock, or calcium that came out of solution — and your filter will clear it. That's a waiting problem, not a chemistry problem.
• The level dropped. The shock is still being consumed, which means you're fighting something that isn't finished, and clearing the water is the wrong goal right now.

This is the distinction nearly every article on cloudy water misses. They list six causes without giving you any way to tell which one you have, and the test that separates them takes one overnight.

Two specifics help. Granular cal hypo doesn't dissolve instantly, so some post-shock haze is just the product itself. And if your calcium hardness and pH are both high, shocking can push calcium out of solution, which is the start of scale rather than dead algae.

There's a third case neither branch catches, and it shows up after a busy weekend rather than after anything you added. A pool that's taken a heavy bather load can go dull and milky simply because the chlorine can't keep up with what people brought into it. Sweat, sunscreen, oils and everything else bind the chlorine up faster than it can work through, and what you end up looking at is combined chlorine.

The test for that one isn't the overnight hold, it's combined chlorine — total minus free. And when it's the answer it isn't subtle. The sample colors up on the first or second drop, which tells you the number is high before you've finished counting.

Expect the fix to be bigger than feels reasonable, because clearing combined chlorine means overwhelming it rather than topping up. I've had pools where that meant thirteen gallons of liquid chlorine and twenty pounds of shock to barely get ahead of it. It's also the same chemistry that shows up further down this page as stinging eyes, just seen from a different angle.

Either way, hold off on clarifier until you know which branch you're on.

The Water Is Clear, but There Are Spots on the Surface

This is the combination that confuses people most, and it's the one where the diagnosis actually matters, because the two candidates need different work.

Clear water with something growing on the surfaces means the problem isn't floating. Brush one of the spots hard and watch what happens. That single action separates almost everything.

Mustard algae lifts off the wall into a dusty yellow or tan cloud and drifts away, then settles back into the same places within a day or two. It looks like sand or pollen that won't vacuum up. The full guide to defeating it is here.

Black algae doesn't move at all. You can put real weight behind the brush and the spot will still be there when the water settles. Get close and you'll see it isn't truly black either, it's a very dark green that reads as black from a few feet away. It comes back in exactly the same square inch rather than nearby. Here's what actually kills it.

Both of them can live in a pool whose chlorine tests perfectly fine, which is why clear water is no reassurance at all.

There Are Stains That Won't Brush Off

A stain sits in or under the surface rather than on it, so brushing does nothing and the shape doesn't change from week to week. There are three usual families, plus a fourth answer that isn't a stain at all.

Start with your hand, before you test or buy anything. Run it over the mark. If you feel a dent or a chip rather than smooth plaster, that's surface damage and no chemical will fill it back in. People spend entire seasons shocking a spot like that, because a shadow sitting in a low place looks dark from the deck.

If it's smooth, two spot tests will usually name it. Hold each one against the mark for a minute or two and watch.

• A vitamin C tablet lightens it — metal staining
• A chlorine tablet lightens it — organic staining
• Neither does anything, and it's rough — scale, which your hand already caught

So the real question after a positive metal test is where it came from:

• Your fill water, or a well
• A copper heat exchanger in the heater
• A copper-based algaecide
• Low pH pulling metal out of your own equipment

There's White Crust on the Tile Line

Calcium scale shows up as a white or grey crust along the waterline, and the giveaway is that it's rough. Your fingertips catch on it. A stain is flat, scale stands off the surface, and that's the same flat-versus-raised distinction that separates staining from late stage black algae.

The confirming test is a drop of acid on the deposit. Carbonate scale fizzes. If it doesn't, you're likely looking at silicate, which is a different and considerably more stubborn problem.

Now the part that decides whether you ever do this again. Almost every page on scale tells you to lower your calcium hardness, and on most scaling pools that isn't the driver — pH and alkalinity drift are. Water deposits calcium when it's carrying more than it can hold at that pH, so the number that answers this is the saturation index, which combines calcium, pH, alkalinity and temperature into one verdict: scaling, balanced, or etching.

It collects at the tile line because that's where water evaporates and leaves behind whatever was dissolved in it. Scrubbing the band off without changing the water that made it just schedules the job again for next season.

There's Foam on the Surface

Foam is persistent piles of bubbles that build where water is agitated, and it doesn't pop and vanish the way ordinary surface disturbance does. There are two completely different causes, and one observation splits them.

Turn the pump off. If the foam stops being produced, you have air getting into the suction side of your plumbing. Usual suspects:

• A loose pump lid
• A bad lid o-ring
• A low water level pulling air through the skimmer
• A leak somewhere before the pump

If the foam keeps forming with the pump running and lingers on its own, you have a surfactant in the water. Quaternary ammonium algaecides are the usual source by a wide margin — the cheap ones — and they foam by design. Polyquat products are generally better behaved on this. Bather oils, lotions and detergents contribute, and low calcium hardness makes water foam more readily than hard water does.

Worth keeping separate: cloudy water is suspended in the water rather than sitting on it, and scum and biofilm are films rather than bubbles. And defoamer collapses foam within minutes while removing nothing that caused it.

Eyes Sting and Skin Itches

This is the most widely misdiagnosed thing in pool care, and almost everyone has it backwards. Stinging eyes and itchy skin are not a sign of too much chlorine.

Check pH first. That's a correction on the standard internet answer, which jumps straight to chloramines — but water outside roughly 7.2 to 7.6 irritates eyes on its own, regardless of what your chlorine is doing, because it's out of range with the tear film in your eye. It's also the faster of the two things to fix.

Then measure combined chlorine, which is what forms when chlorine binds with sweat, sunscreen, urine and everything else swimmers bring in. Test total chlorine and free chlorine, then subtract. What's left is combined, and even a small amount of it irritates and smells. Most test strips can't give you this at all, which is why most people never find it.

The counterintuitive part is the fix. Clearing combined chlorine takes a hard dose, so the answer is usually more chlorine rather than less. Backing off because your eyes hurt makes the thing hurting your eyes worse.

Chlorine Disappears and You Keep Chasing It

You dose the pool, the number looks right, and by the next afternoon it's gone. This one isn't visible in the water, but it's what many of the problems above eventually trace back to.

The test that answers it is the overnight chlorine loss test, and it works by taking the sun out of the equation. Read your free chlorine at dusk, leave the pump running, add nothing, and read it again at first light. Sunlight can't be responsible for anything that happened in the dark, so whatever disappeared overnight was consumed by something in the water.

Treat the two outcomes as separate diagnoses.

• Almost nothing lost overnight, plenty lost during the day. Your problem is sunlight, which points at your stabilizer level rather than anything living.
• A meaningful amount lost overnight. You have real demand — algae you can't see yet, ammonia, or organic load — and more chlorine on the same schedule won't get ahead of it.

There's a version of this that catches people who dose with a calculator, and it's worth knowing because it looks like a math problem. If a correctly calculated dose keeps landing under its predicted number on the same pool, week after week, the arithmetic probably isn't what's wrong. Something in that water is consuming chlorine, and the gap between what you predicted and what you actually measured is the signal.

There's a case where no dose works at all. A pool that overwintered uncovered can build up enough ammonia that free chlorine reads zero no matter what you add, because it's being converted as fast as it goes in.

If a store offers to test your phosphates at this point, the honest answer is that phosphate level predicts very little as long as your free chlorine is genuinely where it should be for your stabilizer. When it is, and algae keeps coming back anyway, that's the case actually worth testing for.

The Handful of Tests That Settle Most of This

Nearly every problem on this page is decided by one of five things, and only one of them costs money.

• Free chlorine read against your stabilizer. Not chlorine on its own. The same 3 ppm is plenty in one pool and nearly useless in another, depending on how much cyanuric acid is holding it.
• pH. It decides how much of your chlorine is in the form that actually kills things, and it's the cheapest correction available for almost anything.
• Calcium hardness. The deciding number for scale, for cloudiness with no other cause, and for foam at the low end.
• The overnight chlorine loss test. The only way to separate sunlight from demand, and it costs nothing but patience.
• Your hand. Stain or damage, algae or scale, flat or raised. Two seconds and it settles questions a test kit can't.

Almost Everything Here Is the Same Problem

Read back through this page and you'll notice how many of these trace to the same root.

Green water is chlorine that went to zero. Both kinds of algae on your surfaces needed a window where chlorine wasn't doing its job. Chloramines are chlorine that got used up and never replaced. Chlorine demand is the measurement of that happening. Even scale and staining, which aren't chlorine problems, come from numbers that drifted over months while nothing looked wrong.

That's the honest summary of pool care. It isn't a knowledge problem and it's rarely one bad decision. A pool drifts slowly, every reading looks acceptable on the day you take it, and nothing tells you that the same acceptable number has been quietly getting worse since May.

A single test can't show you that. A record of your tests can.

That's what I built FlowBeacon for. You log your readings, it keeps them, and it shows you which direction they're heading instead of making you remember what your chlorine was doing three weeks ago. It's free to start, and it doesn't throw away your history right when the history becomes the useful part.

Everything on this page is fixable. The better outcome is catching the drift before you have to fix anything.