What Pool Pros Need to Know About Mustard (Yellow) Algae

Yellow algae—aka mustard algae—is a chlorine-resistant strain that thrives in warm, sunny climates. It shows up as a yellow or brownish film on pool walls, often in shady or low-circulation areas. And unlike green algae, it doesn’t go down easy.

Here’s what makes it especially frustrating for service pros:

1. It Survives Dry Conditions

Mustard algae can live outside the water. That means it can cling to swimsuits, floats, poles, vacuums, brushes—even your truck—and still stay alive. Just because a pool looks clean doesn’t mean the algae is truly gone.

2. It Spreads Easily Between Pools

If your equipment isn’t fully sanitized between stops, you’re at risk of transferring it from one account to the next. Make sure that your home owners know to sanitize their:

• Shared floats and toys
• Unwashed swimsuits
• Contaminated nets, hoses, vac heads, or brushes

Even a quick skim with a dirty net can transfer it to a new otherwise healthy pool.

3. It Can Reappear After Treatment

Shock alone won’t always cut it. Unless you address the root cause and prevent regrowth, it’s coming back—usually stronger than before.

A Smarter Way to Keep Algae in Check

If yellow algae is a recurring headache, there’s a way to stop treating and retreating. APi’s E-Z POOL® isn’t just a fix—it’s your frontline go-to defense against yellow / mustard algae.

This all-in-one blend makes weekly maintenance easier while helping prevent yellow algae from taking hold in the first place. You get:

• A strong copper-based algaecide that holds up in tough conditions
• Phosphate control to cut off algae’s food supply
• Built-in clarifier, scale inhibitor, and conditioner
• No CYA, so your chlorine stays effective
• Full compatibility with all sanitizing systems—salt, UV, ozone, MPS
  
  

For yellow or mustard algae, simply double your traditional dose. Two scoops instead of one for either salt or non-salt pools and your water will be clear, stable, and under control in no time. No guesswork. No comebacks.

IMPORTANT: Keep in mind that you need to treat all of your equipment and tools as well. Mustard algae can survive dry outside of the pool. This means that it can be easily transferred between bodies of water. You’ll want to make sure to use the pole, nets, brushes, etc in the water after applying the E-Z POOL treatment. 

What About Sodium Bromide?

If you’ve traditionally reached for sodium bromide when yellow algae hits, you’re not alone. It’s been the industry’s quick fix for decades.

But new EPA regulations have changed that—especially for outdoor pools and systems like salt, ozone, and UV. Using sodium bromide in those setups is now a violation of federal law due to it being potentially cancerous.

⚠️ Yes, it’s still on the shelf. No, it’s not legal to use in those cases.
You can read the full breakdown here if you want the science, the policy, and the full list of what’s allowed and what’s not. It’s worth the read.

Bottom line: don’t risk your reputation or your customers’ safety over a product that doesn’t currently fits the rules.