You walk outside, see your pool is a swamp, and reach for the shock, but not all algae responds to the same fix. Treating black algae like green algae, or ignoring mustard algae hiding in your filter, is how pools stay green for weeks instead of days.
How to tell green, yellow/mustard, and black algae apart
The color is the obvious starting point, but texture and location tell you more.
Green algae
Green algae is the most common type San Diego pool owners deal with. It turns your water cloudy or pea-soup green, coats the walls in a slippery film, and floats freely in the water column. It spreads fast, a pool can go from clear to green in 48 hours when temperatures climb and chlorine drops. You’ll usually smell it before you see the full bloom. It’s the easiest type to kill and the one covered in more detail in our post on why your pool turns green.
Yellow/mustard algae
Mustard algae is sneaky. It looks like dirt, sand, or pollen sitting on your pool floor or clinging to the shaded walls and corners. Brush it away and it disperses into the water, then settles right back within hours. That behavior is the tell. It’s a yellowish-green or brownish-yellow color and doesn’t cloud the water the way green algae does. It’s chlorine-resistant, which is why standard shock doses often don’t finish the job. It also hitchhikes on swimsuits, toys, and pool brushes, reinfecting the water after you’ve treated it.
Black algae
Black algae looks like dark blue-green or black spots embedded in your plaster, grout lines, or concrete. It doesn’t float. It doesn’t brush away easily. Those spots are colonies with a tough outer layer that shields the cells underneath from chlorine. Beneath the surface, root-like structures called rhizoids anchor into porous plaster. That’s why superficial treatment leaves survivors that regrow within days. Black algae almost never affects vinyl or fiberglass pools, it needs porous material to grip. If your pool has older plaster or exposed gunite, you’re more vulnerable.
Why each type takes a different chemical approach
One shock treatment doesn’t fit all three. The biology is different, which means the chemistry has to be too.
Green algae is the least resistant. It hasn’t developed the cell-wall defenses the other types have. A high-dose chlorine shock, typically raising free chlorine to 10-20 ppm depending on severity, kills it directly. A standard quat-based algaecide as a follow-up helps prevent regrowth.
Yellow/mustard algae has developed real chlorine tolerance. Standard shock at normal doses won’t fully penetrate the colonies. You need to raise free chlorine to 30 ppm in many cases, and you need a polyquat 60 algaecide, not a cheap quat formula. The polyquat molecule is larger and more persistent. Equally important: anything that touched the pool water during the outbreak, floats, brushes, goggles, swimsuits, needs to be decontaminated with a diluted bleach solution before going back in the pool. Skipping this step is the number-one reason mustard algae keeps coming back.
Black algae requires a different attack entirely. Copper-based algaecides penetrate the protective outer layer more effectively than chlorine alone. But chemistry alone won’t do it, you have to physically break the outer layer first with aggressive brushing (more on that below). The shock dose needs to be high, and you’ll likely need to repeat the process over 2-3 days. Some black algae spots in rough plaster are deep enough that the only permanent fix is pool resurfacing, once rhizoids get into deteriorated plaster, no amount of chemical treatment fully eliminates them.
A note on cyanuric acid: San Diego pools often run high stabilizer levels because of the sun. If your cyanuric acid is above 80 ppm, chlorine’s killing power drops dramatically regardless of how much you add. Check stabilizer levels before treating any algae outbreak. Our post on cyanuric acid being too high covers the correction process.
The brushing-shocking-filtering sequence that actually works
Order matters. Doing these steps out of sequence reduces effectiveness significantly.
Step 1: Brush hard before you add chemicals. For green algae, a standard nylon pool brush works. For mustard algae, brush every surface including steps, ledges, and behind ladders. For black algae, use a stainless steel wire brush on plaster pools. Brush directly into the black spots repeatedly, you’re breaking the protective outer cell layer so the algaecide can penetrate. This step is not optional. Without it, you’re pouring chemicals onto a shell.
Step 2: Balance water chemistry first. pH between 7.2 and 7.4 makes chlorine significantly more effective. High pH (above 7.8) neutralizes a large percentage of your free chlorine before it reaches the algae. Adjust pH and alkalinity before shocking.
Step 3: Apply algaecide, then shock. Add the appropriate algaecide according to label dosing. Then shock the pool, at night if possible, since UV burns off chlorine before it can work. For green algae, 1 lb of calcium hypochlorite per 10,000 gallons is a starting point. For yellow and black, you’re likely doubling or tripling that.
Step 4: Run your filter continuously. Run it 24 hours straight until the water clears. Dead algae cells have to go somewhere, the filter is where they go. For a serious outbreak, set the multiport valve to “waste” for the first backwash to avoid pushing debris back through the system.
Step 5: Backwash or clean your filter. A clogged filter after an algae treatment stops doing its job. Clean or backwash your filter 24 hours after treatment and again after 48 hours if the water is still cloudy. For mustard algae specifically, consider rinsing cartridge filter elements with a diluted bleach solution, algae spores can survive inside the filter media and reinfect the pool.
When DIY won’t cut it, signs you need a pro
Most green algae outbreaks respond to a proper DIY treatment within 3-5 days. Here’s when to stop experimenting and call someone.
You’ve shocked the pool twice and it’s still green or yellow. Two failed treatments usually mean there’s a chemistry problem (high stabilizer, low circulation, a bad filter) that a standard shock routine won’t fix on its own.
Black algae spots are growing back within a week. If you treated the pool and the spots return fast, the rhizoids survived. That means either the plaster is porous enough that surface treatment can’t reach the roots, or the treatment protocol wasn’t aggressive enough. A professional can assess whether resurfacing is necessary.
Your filter is running but the water won’t clear. Sometimes a dead algae bloom needs a flocculent plus a vacuum-to-waste sequence that’s genuinely easier with the right equipment. It can also mean your filter is overdue for a deep clean or replacement.
You’re not sure what type of algae you’re dealing with. Misidentifying mustard algae as green and treating accordingly wastes time and money. A pro can identify it quickly and apply the right protocol on the first pass.
Our green pool recovery service is designed specifically for these situations, pools that have progressed past a simple shock treatment and need a structured recovery process to get back to swim-ready condition.
How to keep algae from coming back in San Diego summer
San Diego’s summer conditions are near-perfect for algae growth: water temperatures regularly hit 82-88°F in outdoor pools, UV intensity is high, and pools get heavy use from June through September. Algae spores are always present, the question is whether your water chemistry gives them a foothold.
Keep free chlorine between 2-4 ppm consistently. Not 1 ppm. Not “close to 2.” Consistent levels are what prevent blooms from starting. This is harder to maintain in summer without either a salt system or more frequent dosing.
Run your pump long enough. A good rule of thumb: 1 hour of pump run time per 10°F of water temperature. At 80°F water, that’s 8 hours minimum. Many San Diego pools run short cycles to save electricity and end up with dead spots where algae gets established.
Brush walls and the waterline weekly. Even with good chemistry, algae attaches to surfaces before it colonizes the water. Weekly brushing disrupts early growth before it becomes visible.
Watch your stabilizer. High cyanuric acid is endemic in San Diego pools because of sun-driven chlorine loss. But above 80 ppm, chlorine’s effectiveness drops sharply. Check it monthly in summer and follow a regular maintenance schedule to stay ahead of chemistry drift.
Clean your filter on schedule. A dirty filter circulates water poorly and can harbor algae spores. For most San Diego pools, that means cleaning cartridge filters every 4-6 weeks in summer, and backwashing DE or sand filters when pressure rises 8-10 psi above clean baseline.
If you’d rather not track all of this yourself, weekly pool service keeps chemistry dialed in through the months when algae pressure is highest.
When to call us
If you’ve treated the pool, run the filter, and still can’t clear the water, or if black algae keeps coming back no matter what you try, it’s time to bring in a licensed pro. Stubborn outbreaks usually point to an underlying chemistry or equipment issue that a single treatment can’t solve. Call us at (760) 642-1256 for a same-day estimate.
Frequently asked questions
What is the hardest type of pool algae to kill?
Black algae is the most difficult to eliminate. It forms a protective outer layer over its cells and anchors root-like structures into plaster and grout. Killing it requires aggressive wire brushing, a triple-dose chlorine shock, and often a follow-up algaecide treatment, sometimes repeated over several days.
Can I use the same algaecide for green, yellow, and black algae?
Not ideally. Quat-based algaecides work fine for green algae. Yellow and mustard algae need a polyquat 60 formula because they've developed resistance to cheaper quats. Black algae responds best to copper-based algaecides used alongside a heavy shock. Always check the label for the specific algae type.
How long does it take to clear a pool after treating algae?
Green algae can clear in 24-72 hours with proper shocking and filtering. Yellow algae usually takes 3-5 days because it hides in equipment and needs multiple treatment passes. Black algae can take a week or longer, and may return if any root structures survive in porous plaster.
Does mustard algae spread to pool equipment?
Yes, and that's what makes it so stubborn. Mustard algae clings to swimsuits, floats, brushes, and filter media. You need to clean all equipment with diluted bleach solution and run your filter on waste after treatment, or you'll reintroduce spores right back into the water.
How do I prevent algae in San Diego's summer heat?
Keep free chlorine between 2-4 ppm throughout summer. Run your pump at least 8 hours a day during peak heat. Brush walls and the waterline weekly. Keep cyanuric acid (stabilizer) under 80 ppm so chlorine stays effective. A weekly pool maintenance routine is your single best defense.
Need professional help in San Diego County?
Splash Pro Pools provides every service in this post. Call for a free quote.