It’s a frustrating situation many pool owners face: you diligently add shock to your pool, hoping for crystal clear water, only to find it turning green. Instead of the sparkling blue oasis you envisioned, you’re left with a murky green pool, and a lot of confusion. Why does this happen? It seems counterintuitive, right? You add chlorine, a powerful sanitizer, yet your pool becomes greener. Let’s dive into the common reasons behind this perplexing problem and what you can do to fix it.
Common Reasons for Green Pool Water After Shocking
The truth is, shocking your pool should help clear up green water, not cause it. If you’re experiencing the opposite effect, it indicates an underlying issue preventing the chlorine from doing its job effectively. Here are the primary culprits:
High pH Level
Chlorine’s effectiveness is significantly impacted by the pH level of your pool water. Think of pH as a scale measuring how acidic or basic your water is. The ideal range for pool water pH is between 7.2 and 7.8, with 7.5 being considered perfect. Within this range, chlorine works most efficiently to sanitize and kill algae.
When your pool’s pH creeps above 7.8, and especially when it hits 8.0 or higher, chlorine becomes sluggish and less potent. While high pH alone is rarely the sole reason for a green pool after shocking, it’s often a contributing factor. If your pH is high, the chlorine you add will be less effective at attacking the algae, and the greenness may persist or even worsen temporarily as the shock reacts with existing organic matter but struggles to fully sanitize.
Insufficient Filter Runtime
Your pool filter is the workhorse of your pool cleaning system. It circulates water, removing debris, algae, and other contaminants, keeping your pool clear and healthy. For the filter to do its job properly, it needs to run for an adequate amount of time each day.
Ideally, your entire pool water volume should pass through the filter at least once every 24 hours, known as “turning over” the water. During the summer months, when temperatures are higher and pool usage is greater, you need even more filtration. Warmer water and more swimmers introduce more contaminants and consume chlorine faster, increasing the risk of algae blooms and green water.
Generally, running your pool filter for 8 to 10 hours per day during the summer is recommended. In the winter, you can often reduce this to 4 to 6 hours. If your filter isn’t running long enough, it can’t effectively remove the microscopic algae and organic matter, leading to green water, even after shocking.
Not Enough Chlorine (or Undersized Dosing)
This might seem obvious, but it’s a surprisingly common mistake. Adding an insufficient amount of chlorine to your pool, especially when shocking, simply won’t be enough to combat an algae bloom and clear up green water.
A frequent cause of under-dosing is miscalculating your pool’s volume. Pools can be larger than they appear, and inaccurate estimations can lead to using too little chlorine. For example, the original article mentions a pool service client whose pool was miscalculated at 8,000 gallons by a previous company, when it was actually 14,000 gallons! This nearly doubled the required chlorine amount, making it impossible for the previous service to properly sanitize the pool.
Always accurately measure your pool’s dimensions to calculate the correct volume and ensure you are adding the appropriate amount of chlorine based on the product instructions and your pool size.
Excessive Cyanuric Acid (CYA) – The Most Likely Culprit
High Cyanuric Acid (CYA) levels are arguably the number one reason for green pools after shocking, and it’s a problem many pool owners are unaware of. CYA, also known as stabilizer or conditioner, is designed to protect chlorine from being broken down by sunlight. It acts like “sunscreen” for your chlorine, preventing it from dissipating too quickly.
However, too much of a good thing can be detrimental. When CYA levels become excessively high, it essentially “locks up” the chlorine, hindering its ability to sanitize and kill algae. Think of it as handcuffs for your chlorine, preventing it from working effectively.
The ideal CYA range is typically between 30-50 ppm (parts per million). When CYA levels approach or exceed 100 ppm, it’s time to address the issue. The only way to lower CYA is by partially draining your pool and refilling it with fresh water.
The CYA-Chlorine Relationship: There’s a crucial relationship between CYA and chlorine levels. For chlorine to be effective when CYA is present, you need to maintain a higher chlorine concentration. A general guideline is to keep your free chlorine level at least 7.5% of your CYA reading.
For example, if your CYA is a high 110 ppm, you would need to maintain a minimum chlorine level of around 8 ppm (110 x 0.075 = 8.25). This is significantly higher than the typical recommended chlorine range of 1-4 ppm for pools without excessive CYA. If you were to maintain chlorine at only 4 ppm with a CYA of 110 ppm, you’d likely end up with a green pool because the chlorine would be too weak to overcome the algae in the presence of high CYA.
How Does CYA Get Too High? The primary source of CYA buildup is through the use of stabilized chlorine products. There are two main types of chlorine:
- Unstabilized Chlorine: Liquid chlorine (sodium hypochlorite) is unstabilized and does not add CYA to your pool water.
- Stabilized Chlorine: Trichlor tablets (3-inch tablets) and dichlor granular chlorine are stabilized and do contain CYA.
Using stabilized chlorine products, particularly for routine chlorination or shocking, gradually adds CYA to your pool water over time. If you primarily use trichlor tablets or granular dichlor, you could be unknowingly increasing your CYA levels with each application. This is why consistent use of stabilized chlorine can eventually lead to CYA buildup and related issues like green pools, even after shocking.
Liquid chlorine is often recommended for regular sanitizing because it effectively chlorinates without adding CYA, helping to prevent CYA levels from becoming excessive. Stabilized chlorine can be useful for specific applications, like feeders or vacation chlorination, but liquid chlorine is generally preferred for daily or weekly sanitization to avoid CYA accumulation.
Resolving Green Pool Water
The key to resolving green pool water after shocking is to identify the underlying cause and address it directly. Here’s a quick guide based on the reasons discussed:
- High pH: Test your pool water pH and adjust it to the ideal range of 7.2-7.8 using pH decreaser (muriatic acid or sodium bisulfate).
- Insufficient Filter Runtime: Increase your filter runtime to 8-10 hours daily during summer and ensure proper water circulation.
- Not Enough Chlorine: Accurately calculate your pool volume and use a sufficient dose of shock based on product instructions and pool size. Consider double-shocking for severe algae blooms.
- Excessive CYA: Test your CYA level. If it’s above 50 ppm, perform a partial drain and refill to lower it. Consider switching to unstabilized liquid chlorine for routine sanitization to prevent future CYA buildup.
Conclusion
Seeing your pool turn green after shocking can be discouraging, but understanding the reasons behind it empowers you to take corrective action. By checking your pH, filter runtime, chlorine dosage, and especially your CYA levels, you can pinpoint the problem and restore your pool to its sparkling blue glory. Remember, maintaining proper water balance and understanding the impact of pool chemicals like CYA are essential for preventing green pool water and enjoying a healthy and inviting swimming environment.
