Cloudy pool water: how to tell the causes apart

What your test says about the cause

• Free chlorine below the minimum for your stabilizer level: this is early algae, before it turns green. The water goes dull first because the chlorine ran out and something started growing. It is the most common cause.
• Every reading in range: the filter is the suspect. Check how many hours the pump runs, whether the cartridge needs cleaning or the filter a backwash, and whether the pressure gauge sits well above its clean reading.
• Calcium and pH both on the high side, or the water balance reading scaling: calcium is coming out of solution as a fine white haze. Bringing pH down toward the low end of its range is the first move.
• A white cloud a day or two after adding cal-hypo or soda ash: that addition is still settling in. It usually clears on its own with the filter running.
• Cloudy after a shock or a clearing that worked: the algae is dead but still in the water, too fine for the filter to catch on the first pass. Keep the filter running and give it time.
• Stabilizer above your pool’s ceiling: so much of the chlorine is held back that the water stays dull whatever you add. Replacing some of the water is the fix, and the app sizes it in gallons.

Tell the app the water is cloudy

When you log a test, the question "How does the water look?" has a Cloudy choice. Pick it. If your free chlorine is under its minimum, the app treats the cloudiness as early algae and gives you a clearing plan, because waiting for green costs days and more chlorine. If your chlorine is fine, it shows a short list of the causes your own readings point to, and that card asks you to buy nothing.

Before you buy a clarifier or a floc

Neither one fixes the cause. A clarifier helps the filter with the last of the haze once the cause is dealt with, and a flocculant is a last resort that needs vacuuming to waste. Test first, run the filter, and see the article below for when each one earns its place.

Start with a water test, not a product

Cloudy water looks the same whatever is causing it, so guessing is expensive. Test the water and let the app read the numbers, because the same reading that names the cause is the one that sizes the fix. A product bought before the test is a product bought for a cause you have not identified yet.

The five causes, in the order they usually turn out to be true

• There is not enough free chlorine for your stabilizer level. This is the most common cause by a wide margin, and it is early algae you cannot see yet. Stabilizer decides how much chlorine your pool needs, so a level that was plenty last month can be short now; the app sets your target against your stabilizer reading and sizes the dose.
• The filter is not keeping up. Too few pump hours, a dirty cartridge, a filter due for a backwash, or air getting into the system will all leave the water hazy while every chemical reading looks fine. Run the filter continuously until the water clears, and clean or backwash it once the pressure gauge sits well above its clean reading.
• Calcium is coming out of solution. When hardness and pH are both on the high side the water can no longer hold its calcium dissolved, and it appears as a fine white cloud, often a day or two after adding cal-hypo or soda ash. Bringing pH toward the low end of its range is the move, and the app tells you when your reading is in that territory.
• Dead algae is still in suspension after a shock that worked. The algae is gone, but what is left is too fine for the filter to catch on the first pass. Keep the filter running and give it time. This is the one case where a clarifier genuinely helps.
• Stabilizer is high enough that no chlorine level is doing much work. Past a certain point most of the chlorine you carry is locked up, and no amount you can safely add gets ahead of it. The answer is replacing some of the water, which the app sizes for you in gallons.

When a clarifier helps

A clarifier clumps particles too fine for your filter into pieces big enough for it to catch. That makes it useful for exactly one of the five situations above — the last of the haze once the cause is already fixed, usually dead algae or fine dust — and no use for the other four. It needs a clean filter running long hours to do anything at all, and it takes a day or two to show a result. It speeds up the end of the job; it never does the job.

Why a flocculant is a last resort

A flocculant is not a stronger clarifier, it is a different tool. It binds everything into a blanket that sinks to the floor, and that blanket has to be vacuumed out to waste — straight to the drain rather than through the filter — so you need a multiport valve and you will lose several inches of water doing it. On a cartridge filter there is no way to vacuum to waste at all, and pushing the sludge through the filter instead packs it solid and leaves the pool worse than it started. It also pulls pH down hard. It earns its place on a heavy debris load when you are set up for it, and almost nowhere else.