Algae doesn't sneak up on a pool. It shows up because something gave it an opening — sanitizer dipped, circulation is weak, stabilizer crept too high, or a spring storm dumped a season's worth of organics into the water overnight. The bloom is the symptom. The opening is the problem, and if you only kill the bloom you'll be doing it again in three weeks.
Here's how algae actually establishes itself, how the three types you'll encounter in the Piedmont differ, and the protocol that genuinely clears a pool instead of turning it cloudy and green for a fortnight.
Why algae blooms
Algae spores are always in your pool. They blow in, they ride in on swimsuits and pool toys, they wash off the deck. A healthy pool kills them continuously and you never see it. A bloom means the kill rate lost the race. The usual culprits:
- Free chlorine dropped. Generally you want 1 to 3 ppm. Sit below that for a hot weekend and you'll see it.
- Cyanuric acid is too high. This is the sneaky one. Excess CYA binds chlorine and blunts its effectiveness, so a pool can read "fine" on free chlorine and still be sanitizing at a fraction of the rate it should. High-CYA pools are the ones that get algae despite good-looking test results.
- Poor circulation. Water that doesn't move doesn't get sanitized. Dead spots are where algae starts.
- Phosphates. Algae food, and it pours in from fertilizer runoff, decaying leaves, and some pool chemicals. Phosphates alone don't cause algae — chlorine failure does — but a high-phosphate pool blooms faster and harder when sanitizer slips.
- Rain and pollen events. This is the Triad's specific problem. A spring pollen dump plus a heavy Guilford County thunderstorm delivers organic load, dilutes your chemistry, and knocks pH sideways in one afternoon. That combination consumes chlorine fast. Test after every big storm, not next Saturday.
Know which algae you're fighting
Green algae
The common one. Free-floating, turns the water hazy then green, and it's the easiest to kill. It responds to sustained high chlorine and brushing. If your pool went green, you almost certainly have this.
Yellow / mustard algae
Dusty, yellowish-tan, clings to shady walls and steps, and brushes away easily — which fools people, because it looks gone and returns the next day. It is notably more chlorine-resistant than green algae and it colonizes surfaces rather than the water column. Killing it takes higher chlorine held for longer, plus decontaminating everything that touches the water: poles, brushes, floats, ladders, swimsuits. Miss the pole and you'll reinfect the pool with your own equipment.
Black algae
Technically cyanobacteria. Dark blue-green spots with a protective outer layer, rooted into plaster, grout lines, and any rough or porous spot. It laughs at normal chlorine. The only thing that works is mechanical: break the protective cap with a stiff stainless steel brush (on plaster only — never on vinyl or fiberglass), then hit the exposed heads with chlorine while they're vulnerable. Repeat over multiple days. Chronic black algae in a rough, aging plaster surface is often a signal the surface itself is done, and the real solution is a resurface, not another gallon of chemicals.
The brush-and-shock protocol
Do these in order. Skipping steps is why people shock three times and stay green.
- Test first, especially CYA. If stabilizer is high, drain and dilute before you shock, or you'll burn chemicals fighting your own water.
- Balance pH into range (generally 7.2 to 7.8, aim low in the range). Chlorine is significantly weaker at high pH. Shocking a pool at 8.2 wastes most of what you add.
- Brush every surface. Walls, floor, steps, behind ladders, the shady corners. Brushing strips algae's protective layer and exposes cells to chlorine. This is the step people skip and it's the one that makes shock work.
- Shock hard, and shock at dusk. Add chlorine to a genuinely elevated level, not a token dose. Do it in the evening so UV isn't destroying it while it works.
- Run the pump continuously. Twenty-four hours a day until the pool is clear. Not on your normal schedule. Continuously.
- Hold the level, don't just hit it once. Retest and re-dose to keep chlorine elevated until the water clears. Green will turn cloudy gray or white before it turns clear — that's dead algae, and it means it's working.
- Brush daily for the duration.
- Filter it out. Once it's dead, the filter has to remove the corpses. A flocculant or clarifier speeds this up, and vacuuming to waste (if you're on a sand or DE filter with a multiport valve) pulls the load out fast.
Dead spots, returns, and eyeballs
If algae keeps starting in the same corner, that corner isn't getting water. Look at your return eyeballs. They should be angled to create a circular flow that sweeps the whole pool, generally aimed slightly downward and in a consistent rotation, so the surface skims and the floor doesn't stagnate. Common failures: all returns pointed straight at the middle, one return aimed at the skimmer (which short-circuits the entire loop), or an eyeball that's been broken off for two seasons and nobody noticed.
Steps, tanning ledges, swim-outs, and the area under the main drain are classic dead zones. So is a pool with a single return on a large free-form shape — that's a design defect, not a maintenance problem, and it's fixable during a renovation by adding returns.
Clean the filter after a bloom
Every dead algae cell you pulled out of the water is now sitting in your filter, along with the organic load it fed on. If you leave it there, you're re-seeding the pool every time the pump runs. After the water clears:
- Cartridge: pull the elements, hose them out thoroughly, then soak in a filter cleaner. Chemical soak, not just a rinse — a rinse doesn't remove oils and dead organics from the pleats.
- DE: backwash, then break the grids down and clean them properly, and recharge with fresh DE.
- Sand: backwash until the sight glass runs clear, then rinse. Consider a deep chemical clean of the sand bed.
Prevention is a runtime and chemistry problem
Algae prevention is not a product. It's two things: enough sanitizer, all the time, and enough water movement, all the time. Keep free chlorine in range every day, not on average. Keep CYA under control. Run the pump long enough to turn the pool over — and if your electric bill is the reason you're not, that's a solvable equipment problem, not a reason to gamble on chemistry. A variable-speed pump lets you run far longer hours at low speed for a fraction of the energy, which is exactly what algae prevention needs. That, along with better circulation and automation, is what our modern pool upgrades are for.
If you're fighting the same bloom every spring, the pool is telling you something about its surface, its circulation, or its equipment. Request a design consultation and we'll look at the actual cause instead of selling you another bucket. Call (336) 471-0103.