How to Get Rid of Pool Algae (and Keep It from Coming Back)

Algae is the most common maintenance problem pool owners face — and one of the most frustrating. A pool that was clear and blue on Monday can turn murky green by the weekend, and once algae takes hold, getting rid of it completely takes more time and chemicals than most owners expect.

This guide covers the practical steps to eliminate pool algae in Northern Virginia, Maryland, and DC — and, just as important, how to prevent it from coming back as summer heat, rain, and pool traffic increase.

Why Algae Grows in Pools

Algae are microscopic organisms that bloom when conditions allow them to: low or depleted sanitizer, high temperatures, nutrients from debris and runoff, and poor water circulation. Outdoor pools are constantly exposed to algae spores through air, rain, leaves, soil, and swimsuits. What prevents them from growing is a properly maintained sanitizer level and good circulation that prevents stagnant areas where algae can establish.

The DMV summer — hot, humid, with frequent afternoon thunderstorms that add organic debris and dilute pool chemistry — is ideal algae-growing weather. This region's pools require more consistent attention during June through August than pools in cooler or drier climates. If you are still getting familiar with sanitizer, pH, alkalinity, and stabilizer, start with our pool water chemistry guide before making major adjustments.

Types of Pool Algae

The type of algae affects how you treat it:

• Green algae — the most common. Turns water green and cloudy when severe; coats walls and floor when mild. Responds well to shock treatment and standard algaecides. The easiest type to eliminate, but also the fastest to return if sanitation lapses.
• Yellow / mustard algae — appears as a yellowish or sand-colored powder on walls and floors, particularly on shaded surfaces. More chlorine-resistant than green algae. Often requires repeated brushing, careful attention to shaded areas, and cleaning accessories that may be carrying spores back into the water.
• Black algae — dark blue-green spots that develop a hard protective coating, making it the most resistant algae type. Almost exclusively found in concrete/plaster pools because it needs a porous surface to root. Requires aggressive brushing to break the protective layer and sustained high chlorine treatment.

Step 1: Test and Adjust Chemistry Before Treating

Don't add algaecide or shock without first testing your water. Specifically:

• Check pH — most pool products work best when pH is in the normal operating range, and many algae treatments call for roughly 7.2–7.4 before shocking. At higher pH, chlorine is less efficient. Adjust pH only according to your test results and product label.
• Check total alkalinity — if it's out of range (many pools target 80–120 ppm), get it in range first so pH is easier to control.
• Check cyanuric acid (CYA/stabilizer) — high stabilizer can make a normal free chlorine reading less effective against algae. If CYA is very high, repeated shock treatments may underperform until the water is diluted and rebalanced.
• Backwash or clean your filter if it hasn't been done recently — you're about to kill a lot of algae, and the dead material will load your filter quickly.

Before You Handle Pool Chemicals

Pool shock, acid, and algaecide are useful tools, but they can be dangerous when handled casually. Read the product label, wear the protective gear the label calls for, and keep children and pets away while chemicals are open.

Never mix pool chemicals together in a bucket or feeder, especially chlorine products and acid. If a granular product label tells you to pre-dissolve, add the chemical to water as directed, not water into a pile of chemical. If the label says to broadcast directly into the pool, follow that instead.

Step 2: Brush the Pool Thoroughly

Before adding any chemicals, brush every surface of the pool aggressively — walls, floor, steps, corners, and especially any areas where algae is visibly concentrated. Brushing breaks down the algae's protective coating or biofilm and exposes it to the chemical treatment. For black algae, this step is non-negotiable — the protective layer must be physically disrupted or the chlorine cannot penetrate it.

Use a stiff wire brush on concrete or plaster pools. Use a soft nylon brush on fiberglass or vinyl to avoid surface scratching.

Step 3: Shock the Pool

Shocking means raising the free chlorine level high enough to kill algae and break down chloramines. The right target depends on the severity of the bloom, your pool volume, and your CYA level. A light green bloom may clear with a lower shock dose; severe green water, mustard algae, or early black algae usually requires a higher dose and repeated testing.

Calculate the shock dose based on your pool's volume and current chemistry. Use calcium hypochlorite (cal-hypo) granules or liquid sodium hypochlorite for shock — both work well when used correctly. Follow the label for your specific product, including whether it should be broadcast directly or pre-dissolved before it goes into the pool. Do not mix different chlorine products or combine shock with acid, clarifier, or algaecide in the same bucket or feeder.

Shock at dusk or after dark. Sunlight rapidly degrades chlorine; evening treatment gives the chemical maximum contact time before the next day's sun exposure.

Run the pump continuously throughout treatment.

Step 4: Add Algaecide

After shocking and circulating the water, add an appropriate algaecide if the bloom calls for it. For green algae, a standard polyquat algaecide (PolyQuat 60 or similar) is commonly used because it is non-foaming. For yellow or black algae, use a product specifically labeled for resistant types and follow its timing instructions; some algaecides should not be added when chlorine is still extremely high.

Do not rely on algaecide alone without chlorine treatment, brushing, and filtration — algaecide is a preventive and supplement, not a standalone treatment for an active bloom. Use copper-based products carefully, especially on plaster or light-colored finishes, because metals can contribute to staining if water chemistry is not managed well.

Step 5: Filter Continuously and Backwash Often

Run the pump 24 hours a day until the water clears. As the dead algae is filtered out, the pressure on your filter will rise — backwash or clean the filter whenever pressure rises 8–10 psi above baseline. Depending on the severity of the bloom, you may need to backwash or clean the filter every few hours for the first day.

If you have a sand filter, consider adding a clarifier or using a flocculant to help it capture fine dead algae particles. If you have a cartridge filter, frequent cleaning throughout the process is essential — dead algae will clog cartridges surprisingly quickly. If circulation is weak, filter pressure behaves strangely, or the water does not improve after repeated cleaning, you may be dealing with an equipment issue that needs pool repair rather than more chemicals.

Step 6: Repeat Brushing After 12–24 Hours

After the initial shock treatment and filtering, brush the pool again to dislodge dead material that has settled or stuck to surfaces. This second brushing significantly speeds up clearing time. With most green algae treatments, the pool should be visibly improving within 24–48 hours and fully clear within 3–5 days with continuous filtration.

Black algae will take longer. Budget a week or more of sustained treatment and multiple brushings, and consider repeating the shock dose after 3–4 days if progress is slow. Black algae that is well-established in plaster pores may require resurfacing for complete resolution — a professional assessment is worth seeking if treatment isn't making clear progress after a week.

How to Prevent Pool Algae Before Summer Traffic Peaks

Algae treatment is reactive. Prevention is less costly, less disruptive, and easier to keep on schedule before July heat, vacations, parties, and afternoon storms start stacking up. Use this prevention pass before the pool is in heavy daily use:

• Verify sanitizer and stabilizer together: Many residential pools do well around 2–4 ppm free chlorine, and pools using cyanuric acid generally need at least 2 ppm. The key is the relationship between free chlorine and CYA; if stabilizer is high, a normal-looking chlorine reading may not control algae well.
• Increase testing before busy weekends: Test more often when water is warm, the pool is getting heavy use, or storms are forecast. Our guide to how often to test pool water gives a practical summer cadence.
• Run the pump long enough to eliminate dead spots: The pump needs to skim, mix, and filter the water each day. In summer, many pools need 8–12 hours of circulation, and hot weather, heavy debris, or weak returns may call for more.
• Brush before algae is visible: Brush steps, benches, corners, ladders, light niches, and shaded walls weekly. These are the areas where circulation is weaker and algae usually establishes first.
• Check and control phosphate levels: Algae feeds on nutrients that enter the pool from dead leaves, fertilizer runoff, and debris. Phosphate remover can help in pools with recurring blooms, but it is not a replacement for sanitizer, brushing, and filtration.
• Respond quickly after rain and heavy use: Heavy rain can dilute sanitizer and wash debris into the pool. Parties and frequent swimming add sunscreen, sweat, and organic load. Test after those events instead of waiting for the next normal service day.
• Keep the weekly basics boring: Skim debris, vacuum, brush, empty baskets, and clean the filter before pressure gets too high. Our weekly pool maintenance checklist covers the routine that keeps algae from getting an opening.

If your pool turns green right after opening, start with the more specific guide to green pool water after opening. If the water is cloudy but not visibly green, the issue may be filtration, pH, sanitizer, or suspended debris rather than an active algae bloom; this cloudy pool guide can help narrow it down.

When to Bring in a Professional

Call for help if the pool is not improving after 48 hours of correct testing, brushing, chlorination, and filtration, or if black algae keeps returning in the same plaster areas. Repeated algae can point to a chemistry problem, poor circulation, a filter issue, hidden debris, or a surface that needs closer inspection.

A professional pool maintenance visit can reset the water, check equipment performance, clean hard-to-reach areas, and build a prevention schedule around the way your pool is actually used.

Need Help Clearing Your Pool?

If you’re dealing with a persistent algae problem that isn’t responding to treatment, or if you want to prevent it from ever becoming a problem in the first place, Beltway Pools’ maintenance team serves Northern Virginia, Maryland, and DC. Contact us to learn about weekly service options for your pool.