What is eating my chlorine in my pool?

Chlorine is essential for keeping your swimming pool clean, sanitized, and safe for swimming. However, over time the chlorine levels in your pool can become depleted. This is often referred to as “chlorine demand” – something is causing the chlorine to be used up faster than normal. But what exactly is causing this increased chlorine demand in your pool? There are a few common culprits.

Main Causes of Increased Chlorine Demand

Here are some of the top reasons why you may be experiencing excessive chlorine loss in your swimming pool:

1. Hot weather

Hot and sunny weather causes an increase in chlorine demand. The sun’s UV rays speed up the evaporation of chlorine, causing it to dissipate faster. High air and water temperatures also accelerate chlorine loss.

2. Heavy bather load

More swimmers in your pool means more contaminants being introduced – sweat, body oils, urine, cosmetics, etc. All of these compounds react with chlorine, causing it to be used up at a faster rate. Running your pool filter for longer periods can help offset some of this increased demand.

3. Rain and wind

Rainwater lowers the concentration of chlorine in your pool by diluting it. Heavy winds can even blow chlorine right out of the water. After a storm, test and adjust chlorine levels accordingly.

4. Improper water balance

If the pH or total alkalinity of your pool water is out of the proper range, chlorine becomes less effective and degrades faster. Maintaining proper water balance is crucial for optimizing chlorine performance and longevity.

5. Algae growth

If algae begins growing in your pool it will rapidly consume large amounts of chlorine. If you notice green, black or mustard yellow algae, treat the pool immediately with an aggressive shock treatment. Brushing walls and vacuuming daily can help prevent algae growth.

6. High cyanuric acid levels

Cyanuric acid (CYA) is used to stabilize chlorine in outdoor pools exposed to sunlight. However, too much CYA can have a negative impact, binding up chlorine and making it less effective. Keep CYA levels between 30-50 ppm.

7. Swimmers reentering pool soon after applying sunscreen

Sunscreens contain compounds that react with chlorine. Waiting at least 15 minutes after applying sunscreen before getting back in the pool allows it to fully absorb into skin. Taking a rinse shower can also prevent sunscreen from immediately binding with chlorine.

8. Debris and organic matter

Leaves, grass clippings and other debris that blows into the pool use up chlorine as it breaks down this material. Regularly skimming the pool surface and running the filter helps keep organic matter at bay.

9. Nitrates and phosphates

High levels of nitrates and phosphates provide nutrients that allow algae and bacteria to grow rapidly, consuming more chlorine. Use a specialty chemical to lower nitrate and phosphate levels. Avoid overusing fertilizers near the pool.

10. Excessive chlorine demand from filling a new pool

When initially filling a new pool, the fresh water has no residual chlorine level built up. Additionally, the new plaster or liner surface releases a lot of chemical compounds that lead to very high chlorine demand. Monitor new pools vigilantly.

Signs of Low Chlorine Levels

Keep an eye out for these signs that the chlorine level in your pool is too low:

– Cloudy water
– Green, black or yellow algae spots
– Increased scaling or etching on pool surfaces
– Bacteria and algae growth on walls and floor
– Unpleasant chlorine odor is gone
– Reduced visibility
– Swimsuits and towels retain odors even after washing
– Eye and skin irritation for swimmers

Testing chlorine levels frequently can help catch decreasing residuals before they turn into full-blown issues. Investing in an electronic chlorine tester takes the guesswork out and gives accurate results.

Shocking Your Pool

If it’s been determined that chlorine demand is the culprit behind decreasing chlorine levels, then shocking your pool is the solution. Shocking refers to adding significant doses of chlorine all at once to quickly destroy contaminants, bacteria and algae while restoring free chlorine residuals.

There are two main types of pool shock:

Chlorine Shock

This is sodium hypochlorite in liquid or granular form. It contains at least 50% available chlorine and raises levels rapidly. Typically fast dissolving and swimming can resume in under 24 hours after shocking. Should not be used with cyanuric acid in the water.

Non-Chlorine Shock

Also called oxidizer shock, this is potassium peroxymonosulfate. Works by releasing oxygen in the water to burn up contaminants. Takes longer to dissolve and be effective than chlorine shock. Swimming must wait at least 24 hours after application. Safe for all pool types.

No matter which type you use, shocking clears up undesirable compounds, brings chlorine residuals back up, and gets your pool sparkling again.

Preventing Excessive Chlorine Demand

While shocking can provide immediate relief when dealing with high chlorine demand, there are also some preventative measures you can take to limit rapid chlorine loss in the future:

– Maintain proper pH – Ideal range is 7.4-7.6. If pH is too high or low, adjust with pH increaser or decreaser.

– Monitor cyanuric acid level – Keep between 30-50 ppm, drain partial water and refill if too high.

– Run filter 2-4 hours daily for clearer water.

– Regularly brush and vacuum pool.

– Apply algaecide according to package instructions.

– Use clarifier to help filter efficiently trap debris and particles.

– Provide ample chlorine feed with tablets in the skimmer or feeder.

– Diligently shock pool on a weekly or biweekly basis to maintain sanitization.

– Limit bather load during peak use times to prevent overcrowding.

– Rinse off sunscreen before entering pool and sit out 15 minutes after application.

– Keep debris out by using tight fitting pool covers and nets.

Finding the Right Chlorine Levels

To keep your pool protected, chlorine levels should be kept within the ideal sanitizing range. Here are the recommended free chlorine residual levels to aim for:

– 1-3 ppm – Routine maintenance level needed to sanitize water

– 3-5 ppm – Prior to heavy bather loads from parties or weekends

– 5-10 ppm – Shocking for cleanup and to boost chlorine

– 10+ ppm – Temporary superchlorination to combat algae growth

The easiest way to monitor chlorine levels is by using test strips that you simply dip in the water. However, these can lack accuracy at times. For the best results, use a digital tester that provides exact free chlorine readings.

Another option is to bring a pool water sample to your local pool supply store. Their professional testing can precisely determine chlorine, pH and cyanuric acid levels so you know exactly what adjustments to make.

Chlorine Safety Tips

While chlorine is beneficial for pools when used properly, it does require some safety precautions:

– Never mix chlorine products together as toxic fumes can result

– Avoid spilling and skin contact with concentrated chlorine

– Store chlorine securely away from children and pets

– Prevent chlorine from getting into eyes by wearing goggles

– Never swallow pool water with high chlorine levels

– Ensure adequate ventilation when handling chlorine products

– Follow all usage and handling instructions provided

– Purchase the correct chlorine product for your pool type

Being informed about using chlorine safely reduces risks and allows you to harness its oxidizing power for sparkling clean pool water.

Alternative Sanitizers to Chlorine

If you want to move away from conventional chlorinated pools, there are a couple alternative sanitizer options:


– Less reactive than chlorine, doesn’t dissipate as rapidly in heat and sunlight

– Provides continuousresidual when generated from a feeder

– Can be used with cyanuric acid for sun protection

– Doesn’t produce strong chemical odor like chlorine


– Introduce metal ions like copper and silver into water instead of chlorine

– Low maintenance and easy to use

– Produce no odors, irritation or eye redness

– Require periodic shocking to maintain effectiveness

– May not be as powerful for sanitizing heavy bather loads

While chlorine is the most widely relied upon sanitizer, bromine and ionizers offer effective alternatives if you prefer less harsh chemicals.

Switching to a Saltwater Chlorine Generator

A growing trend is switching from manual chlorination to using a salt cell chlorine generator. Here’s how they work:

– Sodium chloride salt is added to pool water

– As salt water moves through the cell, an electric charge converts the salt to chlorine

– Chlorine sanitizes the pool, then reconverts back to salt

– No need to buy, handle and store chlorine chemicals

Benefits provided by saltwater pools:

– Low maintenance and convenience

– Gentler on skin, eyes and swimsuits

– Chlorine is steadily produced on demand

– No harsh chemical odors

The only potential downsides are higher upfront installation cost and periodic cell cleaning. Overall, a salt chlorine generator takes the hassle out of maintaining optimal chlorine levels.


Don’t let high chlorine demand leave your pool unsanitary and hazardous to swim in. Identifying what factors are rapidly consuming chlorine and responding with prompt shocking, water balancing, and preventive care will nip the problem in the bud. Paying close attention to your chlorine residuals through frequent testing and adequately feeding chlorine will help maintain the safe and pleasant pool environment your swimmers desire.

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