Overnight Chlorine Loss Test —
Pass or Fail?
Enter your free chlorine readings from dusk and dawn to get an instant pass/fail verdict and find out whether your pool has an algae or organic demand problem.
OCLT result reference
Pool chemistry is not a fixed overnight loss — it depends on your starting FC and what is actually consuming chlorine. Use this table as a guide.
| Overnight FC Loss | Verdict | What it means |
|---|---|---|
| 0–1 ppm | Pass | Normal background demand. No active algae. Safe to swim or close. |
| 1–2 ppm | Borderline | May be normal with low starting FC, wind, or rain. Retest with higher starting FC before concluding. |
| > 2 ppm | Fail | Active organic demand — algae, bacteria, or biofilm. Shock the pool before closing or swimming. |
| FC hit 0 overnight | Invalid / Fail | Pool ran out of chlorine before dawn. Start FC higher (at least 4–6 ppm) and retest after shocking. |
What is an overnight chlorine loss test?
An overnight chlorine loss test (OCLT) is a simple diagnostic that isolates organic chlorine demand from UV-driven chlorine loss. During daylight hours, ultraviolet light destroys free chlorine — that's normal and unavoidable. But after the sun goes down, UV is no longer a factor. Any chlorine lost between dusk and dawn was consumed by something in the water: algae, bacteria, organic biofilm, or combined chlorine demand.
A well-maintained pool in clean condition will lose no more than 1 ppm FC overnight. A pool with active algae — even early-stage algae that hasn't yet turned the water green — will lose 2 ppm or more. The test distinguishes between a pool that looks fine and a pool that actually is fine.
The OCLT is especially important before closing
Closing a pool over active algae is one of the most common causes of a "swamp" at spring opening. The algae continues to grow under the cover all winter — there's no UV to destroy it, but also no functioning chlorine to stop it. Run an OCLT in the weeks before closing while you still have time to shock and retest.
How to run an OCLT
- Check starting FC. You need at least 2 ppm at dusk. Ideally 4–6 ppm. If FC is very low, add chlorine a few hours before sunset and let the pump run. Do not test the same evening you add chlorine — wait until the next night.
- Test at dusk. Wait until the sun has fully set and no direct sunlight is hitting the pool surface. Test and record free chlorine.
- Do nothing overnight. No swimming, no added chemicals. Keep the pump running (it doesn't affect the result, but it keeps the water circulating normally).
- Test at dawn. Before direct sunlight hits the water — ideally before sunrise, or at minimum within 30 minutes of first light reaching the pool surface.
- Calculate the loss. Subtract morning FC from evening FC. That's your overnight loss.
What to do after a failed OCLT
A failed OCLT means the pool has an active organic demand problem that normal chlorine maintenance won't resolve on its own. The treatment is to shock the pool — raise FC to the shock level for your CYA (CYA × 40%, with a floor of 10 ppm FC) and hold it there until the water clears and an OCLT retest passes.
- Brush the pool thoroughly — algae clings to surfaces and is protected from chlorine there. Brushing exposes it.
- Shock to the correct level. For CYA 40 ppm, shock target is 16 ppm FC. For CYA 50, it's 20 ppm. Use the shock calculator for an exact dose.
- Retest FC frequently. If FC drops below the shock target within 4–6 hours, re-dose. Active algae consumes chlorine quickly. Keep dosing until FC holds at shock level overnight.
- Run the OCLT again. Once the water has cleared and FC holds at shock level overnight with no additional dosing, repeat the test. A passing result (≤ 1 ppm loss) confirms the algae is gone.
- Return to normal maintenance. Once the OCLT passes, you can bring FC back down to your normal target range.
Common questions
Can I run the test if it rained overnight?
Heavy rain dilutes the water and also adds organic load (debris, runoff), which can cause a borderline fail unrelated to algae. If it rained significantly, retest on a dry night before drawing conclusions. A light rain — a few minutes of drizzle — is unlikely to materially affect the result.
Does it matter if the pump ran all night?
No. Running the pump overnight does not consume extra chlorine. You should keep the pump running for normal circulation. The result would be similar either way, but running the pump gives you more representative results because water is actually contacting the skimmer, filter, and returns where biofilm can live.
My FC went up overnight — is that possible?
Not under normal circumstances. If your morning reading is higher than your evening reading, it's almost certainly a testing error — either inconsistent technique, reagents, or the test kit. Take both readings again on the next test night with careful technique. This is also a good reminder to keep your test kit reagents fresh and store them away from heat and sunlight.
How often should I run an OCLT?
Most pool owners run an OCLT only when they suspect a problem — hazy water, unexpectedly fast FC consumption, or visible algae starting. The two most useful scheduled times are: at the start of algae season (late spring through summer), and in the weeks before closing to confirm there's nothing growing before you cover the pool for winter.
Track FC every time so trends are visible.
PoolChem Tracker logs every reading with a timestamp. If your FC is declining faster than normal — even without a visible problem — the trend shows up in your history before the OCLT becomes necessary.
