Black spots on the bottom of a pool are usually one of four things: black algae, metal stains from minerals in your water, organic stains from leaves and debris, or structural issues like corroding rebar beneath the surface. Each looks slightly different and requires a different fix, so identifying the cause is the first step toward getting rid of them.
Black Algae: The Most Common Culprit
Black algae is the most frequent cause of small, dark spots on pool surfaces, and it’s also the hardest to eliminate. Despite the name, black algae isn’t technically algae at all. It’s cyanobacteria, a type of microorganism that forms tough, deeply rooted colonies on pool walls and floors. The spots typically start small, roughly the size of a pencil eraser, and feel raised or gritty when you run your hand over them.
What makes black algae so persistent is a multilayered defense system. Each colony is protected by a polysaccharide and protein shell that resists chlorine penetration. Pool professionals often call this the “helmet” or “shield.” Beneath that outer layer sits an additional biofilm composed of proteins, lipids, and carotenoids, all of which further block sanitizers from reaching the living cells inside. Genomic analysis of these organisms has confirmed they carry genes specifically for detoxification and nutrient scavenging, which is how they survive in chemically treated, nutrient-poor pool water.
Porous surfaces make the problem worse. Plaster, pebble, and gunite pools provide the ideal texture for black algae to anchor into. The organisms send root-like structures deep into surface pores, which is why scrubbing the visible spot off the surface doesn’t solve the problem. The roots remain embedded and regrow.
How to Tell Black Algae From a Stain
Before you treat anything, you need to figure out whether you’re dealing with a living organism or a chemical stain. Two simple tests can help you sort this out at home.
- Trichlor puck test (for organic stains): Place a chlorine tablet directly on the spot for a few minutes. If the spot lightens or disappears, the stain is organic, likely from leaves, tannins, or other plant material.
- Vitamin C test (for metal stains): Crush 15 to 20 vitamin C tablets in a sock and hold it against the spot for 3 to 5 minutes. If the spot fades, you’re looking at a metal stain, most likely iron or manganese.
If neither test changes the spot and the mark feels textured or raised, black algae is the likely cause.
Metal Stains From Your Water Supply
Dark purple, black, or gray spots that appear suddenly across the pool floor are often caused by manganese, a mineral commonly found in well water. Manganese enters the pool dissolved and invisible. The moment you add chlorine, the mineral oxidizes, falls out of solution, and deposits onto surfaces as a dark stain. The staining tends to concentrate in grout lines and along the floor where water circulation is weakest.
Iron can also leave dark stains, though iron spots typically lean more brown or rust-colored. Copper stains tend to appear blue-green. If your pool is filled or topped off with well water, metals are a prime suspect anytime dark spots appear shortly after shocking or adding chlorine.
Treating metal stains involves lowering your chlorine level below 1 ppm so the metals stop oxidizing, then bringing your pH down to around 7.2 and total alkalinity to 80 ppm or below. A metal sequestering agent can then suspend the metals in solution so your filter can remove them. It’s worth noting that most sequestering products don’t technically remove metals from the water. They hold metals in suspension temporarily, which means regular reapplication is necessary if your source water contains high mineral levels.
Organic Stains From Leaves and Debris
Leaves, acorns, berries, and tree bark that settle on the pool floor can leave behind dark brown or black marks, especially if they sit undisturbed for a few days. The discoloration comes from tannins, organic compounds released as plant material decays. Certain trees are worse offenders: oaks (because of acorns), holly berries, and maple leaves are common sources.
Organic stains are the easiest type to resolve. Chlorine oxidizes tannins effectively, and enzyme-based pool products speed up the breakdown. Most organic stains improve noticeably within a week of treatment. Prevention is straightforward: skim debris regularly and don’t let leaves sit on the bottom.
Corroding Rebar Beneath the Surface
If you have a gunite or shotcrete pool, black or rust-colored spots can signal corroding rebar (the steel reinforcement bars inside the pool shell). This happens when rebar wasn’t properly encased in concrete during construction. Over time, water works its way through the plaster, reaches the exposed steel, and corrosion begins. The rust then bleeds through to the surface as a dark spot.
Rebar stains are a structural issue, not a water chemistry issue. The visible spot on the surface is always smaller than the area of corrosion underneath. Left unaddressed, the rebar continues to deteriorate and can compromise the pool’s structural integrity. Simply resurfacing the pool won’t fix it either. The corroded metal has to be physically excavated and repaired before new plaster goes on, or the stain will bleed through again.
Cobalt Spots on Fiberglass Pools
Fiberglass pool owners sometimes notice black spots that look like stains but won’t respond to any chemical treatment. These are cobalt spots, and they’re technically not stains at all. Water penetrates the pool’s gelcoat through osmosis, reacts with the resin underneath, and produces a dark byproduct. Think of it as similar to an osmotic blister, except the surface stays flat instead of bubbling up. Cobalt spotting is a manufacturing and materials issue, and it requires professional repair of the gelcoat rather than water chemistry adjustments.
Getting Rid of Black Algae
Because black algae is by far the most common cause of black spots, and the most stubborn, it deserves a closer look at treatment. Standard chlorine levels won’t penetrate the protective shell. You need to physically break through that barrier first.
Start by aggressively brushing every visible spot with a stainless steel brush (on plaster or concrete surfaces) or a nylon brush (on fiberglass or vinyl). This cracks the protective layer and exposes the living cells to chlorine. Next, superchlorinate the pool. For a 25,000-gallon pool, this typically means adding around 3 pounds of granular chlorine shock, broadcast directly into the deep end. Do this at least 24 hours before applying any algae-specific treatment so the high chlorine concentration has time to work on the exposed colonies.
After shocking, bring your free chlorine back to the 2.0 to 4.0 ppm range and ensure your pH and alkalinity are balanced. A black algae-specific product can then target remaining colonies. Expect to brush and retreat multiple times. Because of those deep roots in porous surfaces, a single treatment rarely eliminates every colony.
Preventing Black Spots From Returning
Prevention depends on the cause. For black algae, phosphate control is one of the most effective long-term strategies. Algae need three things to grow: sunlight, water, and nutrients. Your pool provides the first two automatically. Phosphates, which enter the water through fertilizer runoff, decomposing organic matter, and even some municipal water sources, provide the third.
Pools with phosphate levels above 100 parts per billion (ppb) are significantly more prone to algae growth. Above 500 ppb, you’re likely fighting algae all season no matter how much chlorine you add. Keeping phosphates below 100 ppb can reduce algae incidents by up to 70% and makes your sanitizer dramatically more effective. Test your phosphate levels regularly and use a phosphate remover when levels creep up.
For metal stains, the best prevention is treating source water before it enters the pool. If you fill from a well, run a metal test on the source water and use a sequestering agent at the time of filling. Avoid shocking the pool immediately after adding fresh well water, since the chlorine spike is what triggers manganese and iron to oxidize and deposit. For organic stains, consistent skimming and a good pool cover during fall go a long way.