When salt and CYA keep drifting down, owners often assume the problem is chemical demand: add salt, add stabiliser, retest, repeat. In many pools, the more accurate explanation is simpler and more mechanical. Fresh water is getting in when it should not, treated water is leaving when it should not, and the chemistry never gets a chance to settle. One of the most common ways this happens is a pool autofill or top-up valve stuck open. The water level can still look normal, but the pool may be slowly diluting all day and quietly sending excess water out through overflow.
Why low salt and low CYA together matter
Salt and cyanuric acid do not usually fall in lockstep unless the pool is losing treated water and taking in fresh water. Salt is not consumed by sunlight. CYA can reduce over time, but not usually in a sharp, repetitive way without a reason. When both readings keep sliding together, that is usually a water-replacement clue before it is a chemistry clue.
The owner notices that the chlorinator is harder to keep in range, stabiliser was corrected recently but is already low again, and the pool level never seems to drop enough to explain it. That combination is exactly why the autofill issue is often missed.
A stuck-open top-up valve can produce that pattern without any dramatic visual warning. Water enters continuously, the pool reaches its normal operating height, then excess water leaves through the overflow path. From the outside, the pool can look calm and stable while the chemistry is being diluted day after day.
The symptom pattern that points to an autofill fault
Salt keeps falling after correction
You add salt, the chlorinator looks happier for a short time, then the number drifts down again without a big backwash, drain, or known leak repair event.
CYA does not hold where you set it
You bring stabiliser back to a useful range, test again later, and it is already lower than it should be.
Water use is higher than expected
A stuck fill line can waste a meaningful amount of water because the pool may be topping up continuously and shedding the excess through overflow.
The overflow area stays suspiciously wet
A damp grate, pipe outlet, or nearby discharge zone during otherwise dry conditions is a strong clue that the pool is reaching overflow height too often.
In real maintenance work, this is often the point where the story becomes clearer: the chemistry does not behave like a pool with ordinary sanitizer demand, and the water use does not behave like a pool that is sitting still.
Why this gets misdiagnosed so often
- Blaming the salt cell first. A dirty or aging cell can misread salt, but it does not usually explain repeated CYA decline at the same time.
- Assuming sun is “burning everything off.” Sunlight affects chlorine. It does not explain salt loss, and it does not explain a higher water bill.
- Treating CYA decline as a stand-alone issue. Stabiliser can reduce over time, but repeated salt loss plus repeated CYA loss points more strongly to dilution.
- Trusting the waterline too much. A normal-looking level does not mean the fill system is behaving. With a working overflow path, a problem can stay hidden for a long time.
Instead of asking only why the readings are low, ask why corrected water chemistry is not staying in the pool.
How to check whether the autofill is stuck open
If water is still trickling in while the pool is already where it should be, the valve is not shutting off properly.
A float that sticks, drags against the housing, or feels scale-bound can leave the valve partly open. In practice, many valves are not fully jammed open; they just never close cleanly.
Calcium scale, grit, or worn seals can stop a valve from sealing properly even when the float rises.
If it is frequently damp without rain, heavy swimmer splash-out, or recent maintenance discharge, the pool may be topping up past its true set point.
If the chemistry stops sliding so quickly once the supply is off, that strongly supports a pool top up valve problem or a hidden water-loss problem previously masked by the fill line.
One set of readings does not prove much. Repeated dilution while the fill line is active tells you far more than one isolated salt number.
Table — Which symptom set fits best?
| What you notice | What it often suggests | Why it matters | Best next check |
|---|---|---|---|
| Salt keeps dropping after correction | Water replacement, dilution, or real water loss | Salt does not normally disappear on its own | Inspect autofill behaviour, overflow signs, and recent discharge history |
| CYA keeps dropping as well | Fresh water is repeatedly entering the pool | Dilution explains both numbers better than sunlight does | Review fill activity and retest the trend after isolating the line if possible |
| Water bill has climbed | Constant refill may be happening in the background | The pool can waste water without visibly looking low | Check whether the valve is shutting off at operating level |
| Overflow area stays wet | The pool may be filling beyond its intended level | Overflow can hide the true refill problem | Observe the autofill while the pool is already full |
What can look similar but is not exactly the same fault
An autofill stuck open is one strong explanation, but not the only one. The same salt-and-CYA drift can appear when the pool is losing water elsewhere and the top-up system is only masking it.
Hidden plumbing or shell leak
The fill system keeps the waterline looking normal, so the owner sees stable level but unstable chemistry.
Frequent backwashing or waste discharge
Every discharge event removes treated water, so both salt and CYA can fall even with a healthy autofill valve.
Heavy splash-out or water features
Busy family use, spillovers, deck-level water play, and aggressive features can remove more water than expected.
Rain plus poor autofill adjustment
After wet weather, a sticky or badly adjusted float can keep adding water when the pool should already be left alone.
If the valve is physically allowing water in when it should be shut, the fill system itself is faulty. If the valve shuts correctly but runs more often because the pool is truly losing water, the fill system is only masking the underlying loss.
When this is probably not just the autofill
- The autofill shuts off correctly, but the pool still needs water often. That points more toward actual water loss elsewhere.
- Salt is dropping, but CYA is stable. That can suggest a reading issue, a recent partial drain/refill history, or a less consistent dilution pattern.
- CYA is dropping, but salt is not showing much change. That may happen if testing intervals are uneven, additions were recent, or the salt reading is being masked by a sensor issue.
- The overflow area is dry and the fill valve looks normal, but the water bill is still high. At that point, broader plumbing or underground loss checks make more sense.
In other words, the stuck-open valve is a strong lead when the whole pattern lines up, but it should not become a blind spot. The right diagnosis still comes from matching equipment behaviour to chemistry trend.
Repair first, then bring the chemistry back into range
Once you confirm or strongly suspect a pool autofill stuck open, the priority is to stop the unwanted dilution. Adding more salt and stabiliser before that point often wastes time and product.
Scale, debris, or worn shutoff parts are common reasons these valves never seal cleanly.
The pool should rise to its intended level and stop, not continue pushing water toward overflow.
Salt, CYA, free chlorine, pH, and where relevant calcium hardness and alkalinity.
This gives you a real baseline instead of another short-lived correction.
The test of a good repair is that the readings now hold instead of repeating the same slide.
Why chlorine performance suffers even if the salt cell still works
A salt chlorinator depends on stable salt concentration. When the pool is being diluted all the time, output becomes less predictable because the water itself is moving away from the cell’s preferred operating range. At the same time, lower CYA means the chlorine that is produced is less protected from sunlight.
That is why this fault often feels worse than “just low salt.” The owner is dealing with two linked effects:
- salt drops, so chlorine production becomes weaker or less steady;
- CYA drops, so the chlorine that is produced is lost faster in sunlight.
Many people respond by increasing chlorinator percentage, extending runtime, or cleaning the cell again. Those steps may be reasonable later, but they will not solve a pool top up valve problem that is still diluting the water every day.
Table — Autofill fault versus other common explanations
| Possible cause | Salt dropping | CYA dropping | Extra clue |
|---|---|---|---|
| Autofill or top-up valve stuck open | Very common | Very common | High water use, wet overflow area, active fill at normal level |
| Dirty or failing salt cell | May only appear low on the reading | Usually no matching drop | Production issue without the same dilution pattern |
| Normal sunlight and bather load | No direct salt loss | Usually gradual only | Chlorine demand rises, but water use does not explain the pattern |
| Frequent backwash or waste discharge | Common | Common | Loss lines up with maintenance events rather than constant overflow behaviour |
What matters most before you buy more product
When low salt and low CYA keep returning, the key question is not only how much to add. The more useful question is whether the treated water is staying in the pool long enough for any correction to hold. In many cases, a pool top-up valve problem is what keeps the chemistry from stabilising.
FAQ
Yes. Fresh make-up water contains neither pool salt nor stabiliser. If it keeps entering and excess water keeps leaving through overflow or discharge, both values can trend downward together.
Because the pool may simply reach overflow height and shed the extra water. A normal waterline does not prove the fill valve is shutting off correctly.
No. Salt falls when treated water leaves and is replaced by fresh water. That can happen through leaks, overflow, backwashing, waste discharge, or an autofill stuck open.
A dirty or failing cell can misread salt or underperform, but it usually does not explain repeated CYA loss and higher water use at the same time.
It is usually better to stop the dilution first. Otherwise the new stabiliser may simply be diluted out again, and the post-repair reading will be harder to judge.
Inspect the autofill while the pool is already at normal operating level. If water is still entering or the float does not shut off cleanly, that is a strong sign.
Once the unwanted dilution has stopped, retest the full chemistry. In most salt pools, it makes sense to restore salt into the correct operating range first, then adjust CYA and fine-tune chlorine output after circulation.