What Causes Pool Stains and How Do I Remove Them?

Pool stains come in two families. Organic stains come from leaves, berries, algae and other plant matter sitting on the surface. Metal stains come from iron, copper or manganese dissolved in the water and depositing on plaster.

The treatments are completely different and using the wrong one makes things worse, so identify the stain type before you buy anything.

Identify the stain first

The two-part spot test takes five minutes.

1. Hold a chlorine tablet against the stain for about 30 seconds (or place it in a sock and rest it on the spot). If the stain lightens or disappears, it is organic.
2. If chlorine did nothing, press a crushed vitamin C tablet (ascorbic acid) against the stain for 30 seconds. If the stain lightens, it is a metal stain.
3. If neither works, you may be looking at scale, plaster etching or a permanent discoloration rather than a stain.

Color is a useful secondary clue but not proof:

• Brown, green or black in leaf shapes: usually organic
• Rust brown or reddish: iron
• Blue green or teal: copper
• Purple or dark blue black: manganese, or copper reacting with high pH
• Gray or dull white patches: usually scale or etching, not a stain

Removing organic stains

Brush the spot hard first. A lot of what looks like a stain is actually a film that has not been brushed properly.

If brushing does not clear it, balance the water, then shock the pool with chlorine and keep the filter running. Most organic marks lift within a day or two of a proper shock.

For stubborn spots, an enzyme treatment helps break down oils and organic residue that plain chlorine struggles with. Persistent black or dark green spots that keep returning in the same place are usually black algae, which requires brushing with a stiff brush to break the protective layer, then targeted treatment.

Removing metal stains

Metal stains need the opposite of chlorine. Ascorbic acid reduces the metal back into solution, and high chlorine will re-deposit it immediately.

The general approach is:

1. Lower chlorine to near zero and turn off any saltwater chlorine generator. This step is not optional.
2. Lower pH toward the bottom of the acceptable range.
3. Broadcast ascorbic acid or a commercial metal stain remover over the affected area with the pump running. Stains often lift within 30 minutes.
4. Immediately add a metal sequestrant. This is the critical step, because you have just put the metal back into the water and the sequestrant is what keeps it suspended instead of re-staining the plaster.
5. Run the filter continuously for 24 to 48 hours.
6. Slowly bring chlorine and pH back up over several days. Raising chlorine too quickly will stain the pool all over again.

Ascorbic acid consumes chlorine aggressively, so plan for the pool to be unsanitized for a day or two.

Preventing stains from coming back

• Skim floating debris daily. Leaves and petals stain only after they sink and sit.
• Brush weekly so nothing has a chance to bond.
• Keep pH and calcium hardness balanced. High pH pushes metals out of solution.
• Add a sequestrant regularly if your fill water carries metals.
• Never drop metal objects, hairpins, cans or tools into the pool. A single hairpin leaves a rust stain in days.
• Test fill water before a large top off if you have well water.

Stains in the Florida Keys

This is where the local situation really bites. Almost no pools down here have screen cages, so organic material lands on the water constantly and unattended.

Bougainvillea is the worst offender we deal with. Those petals are beautiful and they leach color into plaster remarkably fast, sometimes within a couple of days of settling. Palm fronds and seed pods do the same in brown tones.

Iguana waste is the other genuine local problem. It is dense organic material, it sinks immediately, and left on plaster it produces a dark stain that is much harder to remove than to prevent. Get it out promptly rather than brushing it around.

Our year round sun and heat also accelerate the chemistry, so organic material breaks down and bonds faster here than it would in a cooler climate. The window between debris landing and debris staining is short.

Identify the stain type with the chlorine and vitamin C test before you buy a single product, because the wrong treatment can set a stain permanently.