You walk out to the backyard, coffee in hand, expecting a crystal-clear oasis. Instead, it looks like a swamp. It’s green. It’s murky. It’s gross. Your first instinct is to grab that bag of pool shock and start dumping. But you’re wondering—will shock kill algae, or are you just throwing money into a chemical abyss?
Yes. It will. But honestly, it’s not always that simple.
Shocking a pool is basically an act of chemical warfare. You are introducing a massive, concentrated dose of chlorine (usually calcium hypochlorite or "cal-hypo") to oxidize organic matter. Algae are living organisms. When the free chlorine levels spike high enough, the cell walls of the algae break down. They die. The water turns from a vibrant, nasty green to a cloudy, milky grey. That's the sound of victory. Or at least, the beginning of it.
Why Just Dumping Shock Often Fails
Most people think one bag of shock is a magic wand. It isn't. If you have a full-blown "pea soup" situation, a single dose will do absolutely nothing. The chlorine gets "used up" by the sheer volume of organic debris before it even finishes the job.
You have to hit it hard. I’m talking about "SLAMing" the pool—a term popularized by the experts at Trouble Free Pool. SLAM stands for Shock, Level, And Maintain. It’s not a one-time event; it’s a process. You have to keep the chlorine at a specific "shock level" based on your Cyanuric Acid (CYA) levels until the algae is dead, the water is clear, and you pass an Overnight Chlorine Loss Test (OCLT).
If your CYA is too high, the chlorine you just bought is essentially "locked" and useless. It’s a common trap. You keep adding shock, but the algae keeps laughing at you because your stabilizer levels are at 100 ppm and the chlorine can't actually work. You’ve got to check that balance first.
The Different Stages of the Green Monster
Green algae is the most common. It’s easy to kill if you’re aggressive. It floats or clings to the walls and responds well to standard chlorine spikes.
Yellow or "Mustard" algae is a whole different beast. It looks like pollen or sand at the bottom of the pool. It’s chlorine-resistant. If you treat it like normal green algae, it’ll be back in forty-eight hours. You need to raise your chlorine levels significantly higher—sometimes up to 60% of your CYA level—to actually eradicate it.
Then there’s Black Algae. Honestly? It's a nightmare. It’s not actually algae; it’s a cyanobacteria. It digs roots into your plaster. Shocking alone won't kill it because it protects itself with a waxy coating. You have to scrub those spots with a stainless steel brush to break the "shell" before the shock can even touch the living organism inside.
Will Shock Kill Algae If the pH Is Wrong?
Nope. Well, it might, but you’re making it work ten times harder than it needs to.
Chlorine is incredibly sensitive to pH. If your pH is sitting at 8.0, your chlorine is only about 20% effective. You’re literally wasting 80 cents of every dollar you spend on shock. Before you even think about opening a bag of cal-hypo, get that pH down to around 7.2. At a lower pH, chlorine is much more "active" and aggressive. It’s like sharpening the blade before you start chopping wood.
Does the Type of Shock Matter?
You’ve got options.
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- Calcium Hypochlorite: The most common. It’s strong (usually 65-73% strength). It adds calcium to your water, which is fine unless your water is already "hard."
- Sodium Hypochlorite: This is basically concentrated liquid bleach. It’s great because it doesn't add calcium or CYA. It’s easy to mix.
- Potassium Monopersulfate (Non-Chlorine Shock): Do not use this to kill algae. It’s an oxidizer, not a sanitizer. It’s great for getting rid of "used up" chlorine smells (chloramines), but it won't kill a bloom.
- Dichlor or Trichor bags: These contain stabilizer (CYA). If you use these to shock, you are raising your stabilizer levels every single time. Eventually, you’ll hit "chlorine lock," and your pool will turn green despite having high chlorine readings.
The Physical Work Nobody Wants to Do
You cannot just shock and walk away. The dead algae has to go somewhere.
Your filter is going to get hammered. When the shock starts killing the algae, the water turns cloudy. This is the "corpse" of the algae floating around. You need to run your pump 24/7. Not 8 hours. Not 12 hours. All day and all night.
You also need to brush. This is the part everyone skips. Algae creates a biofilm, a slimy layer that protects the colonies underneath. Brushing breaks that layer and lets the chlorinated water get to the roots. If you don't brush, you're just cleaning the surface.
Check your filter pressure. When it rises 8-10 PSI above your "clean" baseline, backwash it or pull the cartridges and spray them down. If you don't clean the filter, you’re just circulating dead algae back into the pool.
Real World Example: The 10,000 Gallon Swamp
Let's say you have a 10,000-gallon pool. It’s dark green.
- You test your CYA and find it's at 40 ppm.
- According to standard industry charts (like the ones from Bob Lowry or the Pool Chemistry Training Institute), your shock level should be about 16 ppm of Free Chlorine.
- You add roughly two pounds of 73% Cal-Hypo.
- Two hours later, you test. The chlorine is already down to 5 ppm.
- Why? Because it’s busy killing stuff.
- You add more. You have to keep it at 16 ppm until the water is clear. This might take three days and twelve bags of shock. This is where most homeowners fail—they stop too soon.
When Shock Isn't the Answer
Sometimes, the "algae" isn't algae.
If you have metallic tinting—like clear green water that doesn't look "cloudy"—you might have copper in your water. Adding shock to copper-heavy water can actually cause a reaction that turns your hair green and stains your liner. This is called "metal staining." Shocking won't fix it; it’ll make it worse. You need a metal sequestrant for that.
Also, if your phosphates are through the roof, you’re basically feeding the algae a buffet. While phosphates don't cause algae, they make it grow at an insane rate. You can shock the pool clean, but if your phosphates are 2,000 ppb, the second your chlorine dips, the algae will return like a vengeful ghost.
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Common Misconceptions About "Killing" Algae
I hear people say "I put algaecide in, why is it still green?"
Algaecide is mostly a preventative measure. It’s like a vaccine. It’s great at stopping algae from starting, but it’s pretty terrible at killing a massive breakout. Chlorine (shock) is the cure. If you have an active bloom, spend your money on liquid chlorine, not fancy "Green-to-Clean" bottles that are often just overpriced ammonia or copper.
Actionable Steps to Clear Your Pool
If you’re staring at a green pool right now, here is the sequence that actually works.
First, clean the debris. Get the leaves out. Chlorine wasted on a pile of rotting oak leaves is chlorine not spent killing algae. Use a leaf rake and get the floor as clean as possible.
Next, balance the pH. Aim for 7.2. Use muriatic acid or dry acid to get it there. Don't worry about Alkalinity or Calcium right now; just focus on pH.
Test your CYA. This is the most important number. If it’s over 80, you might need to drain some water and refill it before you even try to shock. It’s almost impossible to kill algae with high CYA levels without using a ridiculous amount of chemicals.
Hit it with liquid chlorine. I prefer liquid (sodium hypochlorite) because it works instantly and doesn't add extra minerals. Add enough to reach your shock level.
Brush everything. Walls, steps, behind the ladder, the light niches. Everywhere.
Run the pump 24/7. Do not turn it off until the water is clear. Clean the filter daily.
Maintain the level. This is where people mess up. Test the water every few hours if you can. If the chlorine level drops, add more to bring it back up to shock level. You want to keep the "pressure" on the algae until it’s completely gone.
The Final Test. Once the water looks clear, perform an OCLT. Test your chlorine after the sun goes down. Test it again before the sun comes up. If you lose more than 1 ppm of chlorine overnight, something is still alive in there. Keep shocking. If you lose less than 1 ppm, you’ve won.
Once you’re clear, keep your chlorine levels consistent. Algae can't grow in a pool with proper sanitizer levels. It only shows up when we get lazy with the testing kit.
Stop looking for a shortcut. There isn't one. It’s just chemistry and a little bit of elbow grease. Get your test kit out, check your stabilizer, and start the SLAM process. You’ll be swimming in clear water in a few days if you’re disciplined about it.
Check your filter pressure now and see if it's time for that first backwash.