Black Algae in Pool? How to Kill It for Good (Jacksonville)

Black algae in a pool is the one algae problem that does not go away when you dump in a bag of shock. It shows up as small, dark blue-green or black spots about the size of a pencil eraser, usually on a shady wall, a step corner, or along grout lines, and it comes back in the same spots week after week. On the First Coast, where pool water sits warm for most of the year, it is a common headache on older plaster pools. This guide covers how to confirm you have it, how to kill it for good, what it costs if you hand it off, and what it says about the condition of your plaster.

What Black Algae in a Pool Actually Is

Despite the name, black algae is a type of cyanobacteria, not a true algae. That difference matters because of how it protects itself. Each colony builds up layers, with a tough outer coating that shields the living cells underneath from chlorine. It also pushes root-like structures down into the pores of plaster and the grit of tile grout. Kill the top of the spot and leave the roots, and it grows right back from underneath.

That is why black algae behaves so differently from the free-floating green algae that turns a whole pool cloudy. If your problem is green water, the process in our guide to a pool turning green in Jacksonville is the right one. Black algae rarely clouds the water. The pool can look clear and inviting, with dark specks stuck to the surface that simply refuse to leave.

Is It Black Algae, a Stain, or Just Dirt?

Before you spend money on chemicals, make sure you are treating the right thing. Four quick checks sort most cases out:

• Brush it. Dirt and loose debris brush off with a normal pool brush. Black algae barely budges, and if it does lift a little, it looks the same a day later.
• Feel it. Run a fingertip across a spot. Black algae spots are often slightly raised and can feel slick or bumpy. A metal stain is flat and part of the surface.
• Look at the pattern. Black algae shows up as round spots that spread outward over time, favoring shady walls, steps, the area behind a ladder, and tile grout. Stains tend to follow a pattern tied to a source, like a ring at the waterline or a patch near a return jet.
• Hold a chlorine tablet on it. On a plaster pool, holding a trichlor tablet against a spot for a minute or so will usually lighten organic growth. If nothing changes, you may be looking at a mineral or metal stain instead.

If you are still not sure, a local pool pro can usually tell at a glance. It is worth the short visit if the alternative is a week of treatment aimed at the wrong problem.

Why Black Algae Shows Up in Jacksonville Pools

Black algae needs a foothold, and Northeast Florida pools tend to give it one. Warm water most of the year means it never really goes dormant. Our summer rainy season, with afternoon thunderstorms that dilute the pool and wash debris in, drags chlorine levels down right when the water is warmest. Mature live oaks over pools in Mandarin, San Marco, Riverside, and Avondale add shade and constant leaf litter, and shady, low-flow corners are exactly where it starts.

Many pool pros also point to swimsuits, toys, and floats that have been in the ocean, a lake, or the St. Johns River and then go straight into the backyard pool without a rinse. If your family spends weekends at Jacksonville Beach or on the water, rinsing gear before it hits the pool is a cheap habit worth building.

The biggest factor is the surface itself. Smooth, healthy plaster gives black algae little to grab. Rough, pitted, aging plaster, especially the kind you find on pools that are fifteen or twenty years into their current finish, gives the roots endless places to anchor. Keep that in mind, because it comes back up at the end.

How to Kill Black Algae, Step by Step

This is a grind, not a one-day fix. Plan on several days of work and a couple of weeks of watching. The steps below assume a plaster (gunite or shotcrete) pool. Vinyl liner and fiberglass pools need gentler tools, noted where it matters.

1. Balance the water first

Test and bring your pH into the 7.2 to 7.6 range and your alkalinity into normal territory before you start. Chlorine works much harder in water that is not riding high on pH. If your pH keeps creeping up, sort that out first or you will waste a lot of shock.

2. Clean the filter

Backwash a sand or DE filter or pull and hose off a cartridge. You want full flow during treatment, and you are about to put a lot of dead material into the water.

3. Scrub the spots hard

This is the step that decides whether the treatment works. On plaster, use a stainless steel algae brush and scrub every spot until you break the protective layer open. You are not trying to make the spot disappear, you are trying to rough it up so chlorine can get to the roots. On vinyl or fiberglass, stick to a nylon brush, because a steel brush will damage those surfaces.

4. Spot-treat with chlorine

On plaster only, rub a trichlor tablet directly on each spot after brushing. Hold it on for a bit. Do not do this on vinyl, and do not leave tablets sitting on the floor of any pool, because concentrated chlorine will bleach or etch the surface.

5. Shock heavily

Add a strong dose of shock, much more than a routine weekly amount, with the pump running. Most treatment plans call for raising the chlorine well above normal and holding it there. Shock at dusk so the sun does not burn it off. Keep swimmers out until chlorine drops back to a normal level.

6. Add an algaecide made for black algae

Follow the label exactly. Some black algae products are copper-based, and copper can stain plaster if your water balance is off or you overdose, so read the label and use it carefully on light-colored finishes.

7. Run the pump around the clock, then repeat

Keep circulation running 24 hours a day for several days. Brush the spots again every day. Clean the filter when pressure climbs. Expect to repeat the scrub and spot treatment a few times before the spots stop coming back.

8. Clean everything that touched the water

Wash the pool brush, vacuum head, hoses, toys, floats, and swimsuits. Black algae can ride back into a clean pool on gear that was never treated.

What It Costs to Treat Black Algae

Doing it yourself mostly costs time and chemicals. A stainless brush, extra shock, trichlor tablets, and a bottle of black algae treatment typically run somewhere around $75 to $200 depending on pool size and how many rounds you need.

Hiring it out varies more. A pool company treating a mild case during regular service may add a modest charge on top of your weekly rate, while a heavier case that needs multiple visits, extra chemicals, and a filter clean can run a few hundred dollars. What moves the price:

• How many spots and how deep they have rooted
• Pool size, because shock and algaecide are dosed by gallons
• Surface type and condition, since rough plaster takes more rounds
• Whether the filter needs a full teardown clean
• Whether you are already on a weekly service plan

Prices vary across Duval, Clay, and St. Johns counties, and the only real number is a written quote after someone looks at your pool. If you are weighing ongoing service against doing it yourself, regular pool maintenance is what keeps chlorine steady enough that black algae never gets started.

When Black Algae Is Really a Plaster Problem

Here is the honest part. If black algae keeps returning to the same rough spots no matter how well you treat it, the water may not be the problem. Worn plaster with pits and exposed aggregate gives the roots a permanent home, and no amount of shock can reach all the way into a porous, failing finish.

Signs that point that way: the floor feels like sandpaper, the finish is flaking or showing gray patches, stains will not brush out, and the pool is well into the life of its current surface. At that point, it is worth reading our breakdown of pool resurfacing cost in Jacksonville so you know what a new finish runs and whether it makes sense for your pool. A smooth new surface is one of the few things that actually ends a long-running black algae fight.

If you are not there yet, stay on it. Keep chlorine steady through the stormy months, brush the shady walls and steps weekly, and treat any new spot the day you see it. Small spots are a weekend job. Established colonies spread across a wall are a much bigger one.

Frequently Asked Questions

Is black algae in a pool dangerous to swim in?

Black algae itself is not generally considered a major health threat, but its presence means your sanitizer is not keeping up, which leaves room for bacteria. The spots can also make steps slick. Most pool pros recommend staying out until treatment is done and chlorine is back to a normal level.

Why does black algae keep coming back after I shock the pool?

Shock alone rarely reaches the roots. Black algae has a protective outer layer and root-like growth that anchors into plaster pores. You have to scrub each spot with a stainless brush on plaster, spot-treat, shock heavily, and repeat for several days. Rough, aging plaster makes it far more likely to return.

Can I use a steel brush on my vinyl or fiberglass pool?

No. A stainless steel brush is meant for plaster and other cement-based finishes. On vinyl liners and fiberglass it will scratch or tear the surface. Use a stiff nylon brush on those pools and choose a black algae product that is labeled safe for your surface.

How long does it take to get rid of black algae?

A mild case often clears in about a week of daily brushing and treatment. A pool with many established spots can take two weeks or more, and some pools need a second full round. Keep watching the old spots for a few weeks after they disappear.

Does black algae mean I need to resurface my pool?

Not always. Many pools clear up with a proper treatment plan and steady chemistry. But if it keeps returning to rough, pitted areas even after good treatment, the finish may be worn out, and a new surface is often what finally solves it.