PoolLeakFix • Leak Pattern Diagnosis

Pool Losing Water Only at Night? Causes and Simple Tests

If your pool seems fine during the day but you wake up to a lower waterline in the morning, that “only at night”
pattern is trying to tell you something. The trick is sorting out whether it’s
nighttime evaporation, pump schedule, heater use, or a leak that shows itself under those
conditions.

Common Reasons a Pool Loses Water Overnight

When loss shows up mainly at night, it’s usually related to one or more of these:

  • Nighttime pump schedule – the system runs mostly at night, so any circulation leak shows up then.
  • Heater use in cool night air – a heated pool or spa in cool air can evaporate much faster after sunset.
  • Spa spillover or features running overnight – warm water pouring or spraying into the air increases evaporation.
  • Evaporation plus an autofill during the day – the autofill may quietly top off daytime loss, so you only notice the drop before it runs again.

More rarely, the pattern can tie into a specific leak that’s only active when a valve or feature is set a certain way at night.

Step 1: Note What Runs at Night vs During the Day

Start by being very clear about what the pool is doing at night:

  • Is the pump schedule mainly overnight?
  • Is the heater set to hold temperature through the night?
  • Does the spa spill over into the pool all night long?
  • Are any fountains, bubblers, or features scheduled to run after dark?
  • Is the autofill topping up mostly during the day?

The more your system is working at night, the more evaporation and leak potential shifts into those hours.

Step 2: Do a Mark Test with Night Focus

To see if the overnight loss is real and not just a rough guess:

  1. Turn off any autofill so it doesn’t mask the drop.
  2. In the evening, bring the water to your normal level and mark the waterline.
  3. Run the pool overnight as you normally do and check the level first thing in the morning.
  4. Measure how far the level dropped compared to the mark.

If the pool consistently drops more overnight than during the day with similar weather, the pattern is worth taking seriously.

Step 3: Use a Bucket Test Overnight

A bucket test overnight tells you how much of that drop is evaporation and how much may be leak-related.

  • Place a bucket of pool water on a step and mark:

    • The water level inside the bucket,
    • The pool water level.
  • Run the system overnight exactly as you normally would.
  • Compare the two drops in the morning.

If pool and bucket drop about the same:
The overnight loss may be mostly evaporation, especially if the water is heated or features are running.

If the pool drops noticeably more than the bucket:
That extra loss is a red flag for a leak that’s tied into how the system runs at night.

How Heaters and Spa Spillovers Affect Night Loss

Warm water in cool night air is a recipe for evaporation, and it’s even stronger when water is moving and aerated.

  • Pool or spa heater on: Big temperature difference between water and air drives evaporation.
  • Spa spillover running all night: Constant thin sheet of warm water pouring into cooler air.
  • Fountains and jets: Water broken into droplets or thin streams with lots of air contact.

For more detail on how heaters change the picture, see
How Much Evaporation a Pool Heater Causes in Winter (Florida Guide).

Separating Evaporation from a Nighttime Leak

After a couple of mark-and-measure nights and a bucket test, you can ask:

  • Are we seeing loss in the rough range you’d expect from heated water and features, or is it clearly higher?
  • Does the pool drop more than the bucket under the same overnight conditions?
  • Do you see any clues around the deck, yard, or equipment pad?

Alongside a high overnight drop, look for:

  • Soft or damp spots outside the deck.
  • Areas that stay wet even when it hasn’t rained.
  • New cracks or settling along the pool edge.
  • Pump issues if it runs at night (pulling air, losing prime).
  • Autofill running a lot to catch up during the day.

Simple Adjustments You Can Try

If tests point more toward evaporation than a clear leak, try:

  • Shortening heater run time or lowering the set point a bit.
  • Turning off spa spillover and features overnight instead of letting them run for background noise.
  • Shifting part of the pump schedule into daytime if you currently run mostly at night.
  • Using a cover or partial cover when the pool isn’t in use, especially on cool nights.

These won’t fix a leak, but they can reduce evaporation if your system is otherwise tight.

When to Call a Leak Detection Pro

It’s time to bring in a leak detection specialist if you see a mix of:

  • Consistent overnight loss that feels high for your setup.
  • Bucket test results showing the pool dropping faster than the bucket overnight.
  • Deck or yard signs like damp soil, soft spots, or cracking.
  • Water bills or autofill activity that are clearly above normal.

A proper leak detection visit will check plumbing, features, and the shell so you know exactly what’s going on
instead of guessing every morning.

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