Why won't chlorine stay in my pool?
You add chlorine, test the next day, and it's gone. Or your FC holds for a few hours and then drops to near zero. This is one of the most common — and most frustrating — pool problems.
The good news: there are only about seven things that cause rapid chlorine loss, and most are easy to diagnose once you know what to look for.
The quick diagnostic
Before working through each cause, answer two questions that narrow the field fast:
| Symptom | Most Likely Cause | First Step |
|---|---|---|
| Chlorine drops during the day, holds at night | Low CYA — sunlight is destroying FC | Test CYA. If below 30 ppm, add stabilizer |
| Chlorine drops overnight (no sun) | Algae or high organic demand | Run the overnight chlorine loss test (below) |
| FC reads OK but pool is green or cloudy | CYA too high — chlorine is locked up | Test CYA. If above 80 ppm, partial drain needed |
| Chlorine disappears within hours of adding | Active algae bloom or extreme demand | Shock to 30+ ppm FC and brush |
| Pool smells like chlorine but FC is low | Chloramine buildup (high CC) | Test TC. If TC − FC > 0.5, shock |
The overnight chlorine loss test (OCLT)
This is the single best diagnostic for rapid chlorine loss. It removes sunlight from the equation so you can see pure chlorine demand.
- At sunset, add enough chlorine to bring FC to your target level based on CYA (or slightly above)
- Test and record FC right after adding
- At sunrise (before the sun hits the pool), test FC again
- Compare: A loss of 1 ppm or less is normal. More than 1 ppm means something is consuming chlorine — usually algae
Why this test matters
Sunlight can destroy 90% of unprotected chlorine in a few hours, which masks other problems. By testing overnight, you isolate biological and chemical demand from UV loss. If you're losing more than 1 ppm in the dark, no amount of daily chlorine will keep up — you need to find and eliminate the demand source first.
The 7 causes (and how to fix each one)
1. Low or no CYA (sunlight destruction)
What happens: UV radiation breaks down free chlorine molecules directly. Without cyanuric acid to shield it, an outdoor pool can lose most of its FC in 2–3 hours of direct sun.
How to diagnose: Test CYA. If it's below 30 ppm (or at zero), this is almost certainly your problem. Also check: does FC hold overnight but drop during the day? Classic CYA deficiency.
The fix: Add cyanuric acid (stabilizer) to reach 30–50 ppm. Add it in a skimmer sock, run the pump 24+ hours, and retest. Once CYA is in range, your chlorine will last dramatically longer. See the FC/CYA chart for exact targets at your CYA level.
2. CYA too high (chlorine lock)
What happens: Paradoxically, too much CYA causes the same symptom. High CYA (80+ ppm) binds free chlorine so tightly that it can't sanitize effectively. Your test shows FC, but most of it is unavailable. Algae starts growing, which consumes the little active chlorine you have, and FC drops.
How to diagnose: Test CYA. If it's above 70–80 ppm and you're struggling to hold chlorine, CYA is likely the root cause. You may also see early algae signs — slimy walls, slightly green tint, or chlorine demand that never satisfies.
The fix: No chemical lowers CYA. You need to partially drain and refill with fresh water. Drain 30–50% depending on how high CYA is, refill, retest. Then switch to liquid chlorine to stop adding more CYA.
3. Algae (the invisible demand)
What happens: Algae is the #1 consumer of chlorine. And by the time you see green water, the bloom is already advanced. Early-stage algae — invisible to the eye — can consume several ppm of chlorine per day.
How to diagnose: Run the OCLT. If you lose more than 1 ppm overnight, algae is the most likely cause. Other signs: slightly slippery walls, a green tint when you look down into the deep end, or FC that drops faster than it should even with adequate CYA.
The fix: Shock the pool. Raise FC to shock level for your CYA (typically 30–40% of CYA). Brush all surfaces, run the pump 24/7, and maintain shock level until the OCLT passes (under 1 ppm loss overnight). This may take 2–5 days for a bad bloom. See our guide to raising chlorine safely for dosing specifics.
4. High organic load
What happens: Chlorine reacts with and is consumed by organic material — sweat, sunscreen, body oils, leaves, pollen, dirt, and anything else in the water. A heavy bather load or a storm dumping debris into the pool can wipe out FC quickly.
How to diagnose: Did chlorine drop right after a pool party, heavy rain, or a wind event that blew debris in? That's organic demand. You may also notice elevated combined chlorine (CC) as the chlorine reacts with nitrogen-based organics.
The fix: Skim and vacuum debris. If CC is above 0.5 ppm, shock to breakpoint to clear chloramines. For ongoing heavy use, increase your daily chlorine dose. Enzyme-based clarifiers can also help break down organic load between shocks.
5. Chloramine buildup
What happens: When chlorine reacts with nitrogen-based contaminants (urine, sweat, ammonia), it becomes combined chlorine — chloramines. This combined chlorine is useless for sanitizing, and the conversion process effectively removes active FC from your pool.
How to diagnose: Test both FC and TC. If TC minus FC is above 0.5 ppm, chloramines are consuming your chlorine. You'll also notice the classic "pool smell" — that harsh chemical odor is chloramines, not free chlorine. See Free Chlorine vs Total Chlorine for the full explanation.
The fix: Shock to breakpoint chlorination — raise FC to 10x your current CC reading (or to shock level for your CYA, whichever is higher). This destroys chloramines and restores your FC. Once CC drops below 0.5 ppm, you're clear.
6. Underdosing
What happens: You're adding chlorine, but not enough to keep up with daily demand. Every pool has a baseline chlorine consumption rate that depends on sun exposure, temperature, bather load, and CYA level. If you add less than what the pool consumes, FC slowly drifts down.
How to diagnose: Track your FC readings over several days. If FC is consistently 1–2 ppm below target by the time you dose again, you're simply not adding enough. This is especially common with tablet feeders set too low or with tablets that dissolve too slowly in cooler water.
The fix: Increase your daily dose. A typical 15,000-gallon pool in full sun may need 1–2 ppm worth of chlorine added per day just to break even. In hot climates with high CYA, you may need more. Log your readings and adjust until FC holds steady between doses.
7. High water temperature
What happens: Warm water accelerates everything — bacteria multiply faster, algae grows faster, and chlorine reacts and degrades faster. A pool at 90°F will consume chlorine noticeably faster than the same pool at 78°F.
How to diagnose: If chlorine loss gets worse as summer heats up and improves when temperatures drop, heat is a contributing factor. It's rarely the only cause, but it amplifies all the others.
The fix: You can't cool your pool (well, you could, but that's not practical). Instead, compensate: raise your target FC by 1–2 ppm during peak summer, test more frequently, and make sure your CYA is in the right range for your climate. Hot climates benefit from CYA toward the higher end of the range (50–60 ppm) to protect chlorine from the intense sun.
Quick-reference: diagnose your chlorine loss
How to keep chlorine stable long-term
Once you've identified and fixed the root cause, these habits will keep FC steady:
- Maintain CYA at 30–50 ppm (60–80 for saltwater). Test monthly
- Use liquid chlorine for daily maintenance once CYA is at target — it adds chlorine without raising CYA further. See Does Liquid Chlorine Raise CYA?
- Shock when CC exceeds 0.5 ppm to clear chloramines before they snowball
- Skim and brush regularly — removing organics before they consume chlorine is cheaper than replacing the chlorine they eat
- Track your readings — a single FC test tells you one snapshot. A trend over time shows you whether chlorine is holding, drifting, or crashing
The FC/CYA relationship is everything
Most chlorine problems come back to one thing: is your FC high enough for your CYA level? At CYA 30, you need FC of at least 2 ppm. At CYA 60, you need 4–5 ppm. At CYA 80+, you need 6+ ppm. If your FC is below the minimum for your CYA, you're effectively running with no chlorine. Use the FC/CYA chart to check.
Stop guessing. Start tracking.
PoolChem Tracker calculates your chlorine target based on your CYA, tracks FC trends over time, and alerts you when readings drop — so you catch problems before they become algae.
Keep reading
- FC/CYA Chart: How Much Chlorine You Need — find the right FC target for your stabilizer level
- Pool Chlorine Too Low? Causes and How to Fix It — the companion guide to low FC readings
- How to Raise Pool Chlorine Safely — exact dosing for liquid chlorine, cal-hypo, and tablets
- Pool Chlorine Levels Chart — what your FC, TC, and CC readings mean
- Does Liquid Chlorine Raise CYA? — which chlorine types add stabilizer and which don't
- Pool Water Green but Chlorine Is High? — when the problem is CYA masking your chlorine
- Free Chlorine vs Total Chlorine — understanding the gap that signals chloramine buildup