The stubborn algae that laughs at normal chlorine — and how to actually kill it
If you’ve noticed dark black or navy patches forming on your pool floor, steps, or walls that won’t budge no matter how much chlorine you add — you’re almost certainly dealing with black spot algae. It’s one of the most frustrating pool problems an Australian homeowner can face, and it requires a completely different approach to regular algae treatment.
This guide covers everything you need to know: how to identify it correctly, why it’s so resistant to normal treatment, and the exact step-by-step method to eliminate it for good.
⚡ Quick Reference Guide
Already know what you’re doing? Use this table to guide you through the process. First time? Read the full guide below before starting.
| Step | Action | Pump | Time | Notes |
|---|---|---|---|---|
| 1 | Test and adjust water chemistry | Running | 15 mins | Lower pH to 7.0–7.2 before starting |
| 2 | Scrub ALL black spot with stainless steel brush | Running | 20–40 mins | Must break through protective layer — scrub hard |
| 3 | Turn pump off | OFF | — | Water must be still for granular to settle |
| 4 | Apply trichlor granular directly onto scrubbed patches | OFF | 5 mins | Aim carefully — granular must land on the algae |
| 5 | Leave system off | OFF | 1 hour | Do not disturb water during this time |
| 6 | If applicable – Treat wall patches with a trichlor tablet | OFF | 5 mins | Wear gloves — trichlor is highly acidic |
| 7 | Turn pump on, run continuously | Running | 24 hours | Do not turn off during this period |
| 8 | Brush pool, clean filter | Running | 20 mins | Backwash and rinse media filter or hose cartridges |
| 9 | Inspect and repeat if needed | Running | — | Most cases need 1–3 treatment rounds |
| 10 | Rebalance water chemistry | Running | 15 mins | pH, alkalinity, and chlorine will all need adjusting |
What you’ll need at a glance:
- ✅ Stainless steel pool brush (fits standard pool pole)
- ✅ Trichlor granular chlorine (black spot granular)
- ✅ Trichlor chlorine tablets (for wall treatment if applicable)
⚠️ Do not substitute trichlor granular with regular pool shock. Regular shock dissolves too quickly and will not work. See the FAQ section below for more detail.
What Is Black Spot Algae?
Black spot algae (also called black algae) is a cyanobacterium — technically closer to bacteria than true algae, which is exactly why it behaves so differently to green or yellow algae in your pool.
It is widely considered the most difficult pool algae to treat. Unlike green algae which floats freely in your water, black spot algae physically anchors itself into your pool surface and builds a hardened protective shell over the top. This outer layer is almost completely resistant to the active chlorine circulating in your pool water — meaning your normal sanitisation system simply cannot reach the organism underneath.
Left untreated, black spot algae will spread and can eventually cover entire walls and the pool floor. It also makes pool surfaces extremely slippery, creating a genuine safety hazard.
How to Identify Black Spot Algae
Before starting any treatment, it’s important to confirm you’re actually dealing with black spot algae and not metal staining or dirt. Misidentifying the problem leads to wasted time and money.
Black spot algae looks like:
- Dark black or navy blue-black spots or patches
- Similar in appearance to black mould you’d find in a bathroom
- Raised slightly from the pool surface — not flat like a stain
- Slippery and slightly slimy to the touch
Where it typically appears:
- Steps and ledges — areas with lower water circulation are where it takes hold first
- Corners and crevices — anywhere water flow is reduced
- Shaded areas of the pool — black spot thrives with less UV exposure
- In severe cases it can spread across the entire floor and walls
How to tell it apart from metal staining:
- Metal staining is flat and smooth — it does not feel raised or slimy
- Black spot algae has a slightly fuzzy or rough texture and feels slippery
- Metal staining won’t scrub off easily with a brush — black spot algae will partially scrub away, revealing a lighter area underneath
- If in doubt, try the Vitamin C test: press a Vitamin C tablet against the stain for 30 seconds. If the dark colour lifts, it’s metal staining. If nothing happens, it’s algae.
What it is NOT:
- Green algae (which turns water cloudy and green)
- Yellow/mustard algae (which is powdery and brushes off easily)
- Dirt or debris (which vacuums away)
- Iron or copper metal staining (which is flat and non-biological)
Why Black Spot Algae Is So Hard to Kill
Understanding why black spot is so resistant helps you understand why the treatment method below works.
Black spot algae grows in three layers:
- The root system — it physically embeds its roots into your pool surface, whether that’s fibreglass, concrete, or tile grout. This is what makes it so persistent — even if you kill the visible algae on top, the roots can regenerate.
- The protective outer layer — over the root system, black spot algae grows a hardened, waxy outer shell. This layer is highly chlorine-resistant. The active chlorine circulating in your pool water simply cannot penetrate it in normal concentrations.
- The visible surface — the dark patches you can see on your pool surface.
Standard pool chlorination kills free-floating algae by direct chemical contact. But with black spot algae, the protective shell prevents that contact from ever reaching the organism. This is why pools with perfectly balanced chemistry and correct chlorine levels can still have black spot algae — the chemistry is irrelevant if it can’t get through the outer layer.
The solution is to physically destroy that protective layer first, then apply a slow-acting, concentrated chlorine directly onto the exposed roots before they can rebuild their defences.
What You’ll Need
- Stainless steel pool brush — this is non-negotiable. A standard nylon brush will not penetrate the protective layer. A stainless steel brush attachment fits onto your standard pool pole and is available at pool shops and Bunnings for around $20–$40.
- Trichlor granular chlorine (black spot granular) — a coarse, slow-dissolving granular chlorine specifically designed for this treatment. Available at pool shops. Do not substitute with regular pool shock or liquid chlorine.
- Trichlor chlorine tablets — for treating black spot on walls where granular won’t sit.
- Algaecide (black spot specific or polyquat 60) — to assist treatment on walls and as a preventative measure.
- pH Down (sodium bisulphate or hydrochloric acid) — lower pH assists the treatment.
- Gloves — trichlor is highly acidic. Wear gloves if your skin is sensitive, and avoid direct contact with your eyes.
- Pool test kit or strips — to check and adjust water chemistry before treatment.
Step-by-Step Black Spot Treatment
Step 1: Test and Adjust Your Water Chemistry
Before starting treatment, test your pool water and adjust:
- pH: Lower your pH to between 7.0 and 7.2 — slightly lower than the normal range of 7.2–7.6. Lower pH significantly improves the effectiveness of chlorine and helps the treatment penetrate faster. Note: trichlor granular itself lowers pH, so don’t over-adjust.
- Chlorine: Ensure your chlorine is at a normal level (2–3 ppm) before you begin. You don’t need to superchlorinate at this stage.
- Alkalinity: Keep between 80–120 ppm — low alkalinity will make pH unstable and harder to manage during treatment.
Step 2: Scrub the Black Spot With a Stainless Steel Brush
This is the most important step in the entire process — and the one most people skip or do incorrectly.
Using your stainless steel brush attached to your pool pole, scrub every visible black spot patch vigorously. You are not trying to scrub the algae off the surface — you are trying to break through and destroy the hardened protective outer layer.
Scrub hard and repeatedly. You’ll notice the black patches may lighten slightly or develop a rougher appearance — this means you’ve broken through the shell and exposed the roots underneath.
Key points:
- Do every single patch — don’t skip any, no matter how small
- Use firm, circular scrubbing motions
- For steps and flat surfaces, press down firmly
- For walls, scrub as aggressively as the angle allows
- The pump can be running during this step
Do not proceed to the next step until every visible black spot has been scrubbed.
Step 3: Turn the Pump Off
Once all black spot patches have been scrubbed, turn your pool pump off.
This is critical. You need the water to be as still as possible so that when you apply the trichlor granular in the next step, it sinks directly onto the exposed algae and sits there — rather than being swept away by water circulation before it has time to work.
Step 4: Apply Trichlor Granular Directly Onto the Black Spot
With the pump off, take your trichlor granular chlorine and carefully sprinkle it directly onto every scrubbed black spot patch.
The goal is to get the granular sitting directly on top of the exposed roots. Trichlor is coarse and dense — it will sink and settle on the affected areas rather than dissolving instantly like regular chlorine. This slow-dissolving action means it maintains prolonged, concentrated contact with the algae roots, which is what kills it at the source.
Application tips:
- Use a cup or scoop for controlled application
- Aim carefully — you want the granular on the algae, not scattered randomly
- Apply generously — don’t be conservative
- For large patches, make sure the granular covers the entire affected area
For black spot on walls: Granular won’t sit on vertical surfaces — it will simply fall to the floor. For wall treatment, see Step 6 below.
Step 5: Leave the System Off for One Hour
Leave the pump off and the trichlor granular sitting on the algae for a minimum of one hour. This gives the slow-acting granular time to penetrate the exposed roots and begin killing the algae at the source.
Do not brush the pool or disturb the water during this time.
Step 6: Treating Black Spot on Walls
For black spot growing on pool walls where granular cannot sit, you have two options:
Option A — Algaecide treatment: With correct water chemistry (proper chlorine levels and pH between 7.0–7.2), add a black spot specific algaecide or polyquat 60 algaecide to your pool. After scrubbing the walls to expose the roots, the combination of correct chemistry and algaecide will slowly weaken and kill the wall algae over several days. This is slower but effective for wall treatment.
Option B — Trichlor tablet scrubbing: Take a trichlor chlorine tablet and, wearing gloves, scrub it directly onto the black spot patches on the walls. The tablet will slowly dissolve and deposit concentrated chlorine directly into the algae. This is particularly effective for stubborn wall patches that aren’t responding to algaecide alone.
⚠️ Always wear gloves when handling trichlor tablets or granular — it is highly acidic and can irritate skin. Keep away from eyes.
Step 7: Run the Filter for 24 Hours
After the one hour treatment period, turn your pump back on and run it continuously for 24 hours straight.
This circulates the dissolved trichlor through the entire pool, maintains chemical contact with any remaining algae, and begins filtering out dead algae matter.
Step 8: Brush and Clean the Filter
After the 24-hour pump run:
- Brush the entire pool again — walls, floor, and steps. This dislodges any dead algae that has loosened during treatment.
- Clean your filter thoroughly:
- Sand/media filter: backwash until water runs clear
- Cartridge filter: remove and hose down cartridges
Dead algae matter will have accumulated in your filter — a thorough clean here significantly improves water clarity.
Step 9: Assess and Repeat if Necessary
Black spot algae often requires more than one treatment — particularly in severe cases or if the algae has been established for a long time.
After the 24-hour pump run, inspect every area you treated:
- Black spot is gone: Move to Step 10
- Black spot has reduced but persists: Repeat the full treatment — scrub, apply granular, leave for an hour, run for 24 hours
- No change at all: Check that you used genuine trichlor granular (not regular shock), confirm your pH is in the 7.0–7.2 range, and ensure you scrubbed aggressively enough to break through the protective layer
Most cases resolve within one to three treatments.
Step 10: Rebalance Your Water Chemistry
Trichlor granular is highly acidic and will have lowered your pool’s pH and alkalinity during treatment. Once black spot is gone, rebalance your water:
- Test pH — likely to be low, add pH Up (sodium carbonate) to bring back to 7.2–7.6
- Test alkalinity — add alkalinity buffer (sodium bicarbonate) if below 80 ppm
- Test chlorine — top up if needed to 2–3 ppm
- Consider adding a maintenance dose of algaecide to prevent regrowth
For a precise reading, take a water sample to your local pool shop for a full professional analysis — particularly after a heavy treatment like this.
Preventing Black Spot From Coming Back
Once you’ve successfully treated black spot algae, the last thing you want is for it to return. Here’s how to keep it away:
Maintain correct water chemistry consistently: Black spot thrives when chlorine is low or pH is too high (above 7.8). Consistent chemistry is your best defence. Test your water at least weekly during summer.
Improve circulation in problem areas: If black spot keeps recurring in the same spots — steps, corners, or a particular section of the floor — it’s a sign of poor water circulation. Consider adjusting your return jet angles to direct more flow toward those areas.
Brush your pool regularly: A weekly brush of all surfaces, even when the pool looks clean, prevents algae from establishing a foothold. You’re disrupting any early growth before it has a chance to form that protective layer.
Use a preventative algaecide: A monthly maintenance dose of polyquat 60 algaecide keeps algae spores from taking hold. It’s inexpensive and far easier than treating a full outbreak.
Don’t let your chlorine drop to zero: Even a few days of zero chlorine in warm weather is enough for black spot spores to establish. If you’re going away, either ask someone to check the pool or add a slow-release chlorine float before you leave.
Frequently Asked Questions
Can I use regular pool shock instead of trichlor granular? No. Regular granular shock (calcium hypochlorite) dissolves too quickly to maintain the prolonged contact needed to penetrate black spot algae roots. Trichlor granular is specifically designed to dissolve slowly — this slow action is what makes it effective. Using the wrong product is the most common reason DIY black spot treatments fail.
Will black spot algae make my family sick? Black spot algae itself is not directly harmful to swimmers, but it does make pool surfaces extremely slippery which is a safety hazard. It also indicates water chemistry issues that could allow other pathogens to grow. Treat it promptly.
My pool is fibreglass — can I still use a stainless steel brush? This is debated. A stainless steel brush can potentially scratch fibreglass surfaces if used too aggressively. For fibreglass pools, try a hard nylon brush first — if it breaks through the protective layer, use that. If not, use the stainless steel brush carefully and avoid excessive force on flat surfaces.
How long does a full black spot treatment take? The active treatment takes about 30–45 minutes (scrubbing and applying granular), followed by one hour of waiting, then 24 hours of pump running. Total elapsed time is roughly 26 hours per treatment round, with most cases requiring one to three rounds.
Is black spot algae the same as black mould? They look similar but are different organisms. Black mould is a fungus. Black spot algae is a cyanobacterium. The treatment methods are completely different — standard mould removers will have no effect on black spot algae in a pool.
Why does my pool keep getting black spot every summer? Recurring black spot usually means spores are surviving in your filter or on pool equipment between seasons. Give your filter a deep clean at the start of each season, brush all surfaces before opening the pool for summer, and add a preventative algaecide dose. Also check that your return jets are directing flow into the areas where black spot typically appears.
Dealing with a different type of pool staining? Read our guide on how to identify and treat metal staining in your pool for copper, iron, and manganese stain removal.