Black Spots on a Fibreglass Pool: Algae vs Osmosis

A black spot on a fibreglass pool is one of two things: black-spot algae, or an osmotic blister. A fingernail run across the mark tells you which. Black-spot algae sits flush with the gelcoat. An osmotic blister stands proud and catches the fingernail. Black-spot algae is a living organism on the surface, and it clears with brushing, an algaecide labelled for black algae, raised free chlorine and a running filter, repeated until no mark comes back. Every step of that treatment happens with the pool full. In osmotic blistering, water has passed through the gelcoat and reached the laminate underneath, so nothing dosed into the pool water reaches it and the blistering does not reverse. A blistered shell goes to a builder for assessment and resurfacing.

Are the black spots on your fibreglass pool algae or osmosis?

Black spots on a fibreglass pool are one of two unrelated conditions: black-spot algae or osmotic blistering. Black-spot algae is a living organism sitting on top of the gelcoat, so it is a water problem and an owner can treat it with the pool full. Osmotic blistering is water that has crossed the gelcoat into the laminate behind it and pushed the gelcoat outward into a raised dome. Blistering is a shell problem, and no chemical put in the water takes water back out of the laminate.

One check separates the two at the pool edge. Run a fingernail across the mark. Black-spot algae is flush with the surface and the fingernail passes straight over it. An osmotic blister stands proud of the surface and the fingernail catches on its edge.

A mark that is neither may be one of the fibreglass pool stains left by iron, copper or manganese in the water.

An osmotic blister is unchanged where a nylon brush passes over it, because the gelcoat itself has been pushed outward and there is nothing sitting on the surface to take off.

Reading the mark correctly decides which treatment the pool needs, because the two conditions have no treatment in common. Brushing with shock and algaecide clears black-spot algae, and none of that does anything at all to a blister, so an owner who reads blistering as algae keeps dosing a pool whose water was never the problem.

What is black-spot algae and how does it get into a fibreglass pool?

Black-spot algae is an algae that attaches itself to the pool surface and holds on hard once it is there. Michigan State University Extension's pool pest-management manual records black and yellow algae as attaching to the walls, floors and steps of a pool, and states that algae which has adhered to a surface is very difficult to remove.

Black-spot algae takes hold in a fibreglass pool after the water conditions slip. Three conditions let the algae establish. The sanitiser drops below the level that keeps the water hostile to algae. The circulation stalls, so parts of the pool stop turning over. Or a storm drops organic matter into the water.

Black-spot algae establishes first in the shaded, low-flow parts of the shell. Corners, steps, the area under the returns and the waterline are where sanitised water reaches last, and that is where the first spots appear.

On a gelcoat shell the algae sits on top of the surface rather than growing into it. A concrete or plaster pool has a porous finish that algae can root into.

How do you get rid of black spot in a fibreglass pool?

You get rid of black spot in a fibreglass pool by brushing the spots, dosing an algaecide labelled for black algae, raising the free chlorine, running the filter, re-brushing, and repeating that cycle until the marks stop coming back.

Every step in that sequence is done with the pool full, because emptying a fibreglass pool is a structural decision that belongs to the builder and never a step in clearing algae.

1. Brush the spots. Michigan State University Extension's pool pest-management manual states that black algae has to be brushed, because the brush scores the outer wall of the algal cell and leaves it exposed to chlorine, which then kills it. The brush material named in that instruction is written for plaster and concrete. On a gelcoat shell a nylon brush does the same work, because the algae is sitting on the surface rather than embedded in it, and a stainless-steel or wire brush cuts the gelcoat.
2. Add an algaecide labelled for black algae, with the pool still full. Michigan State University Extension gives one instruction for a pool that cannot be drained: dose an algaecide formulated against black, yellow or green algae, and brush physically at the same time so the dead algae comes off the wall. That is the branch a fibreglass pool uses, and it is the only branch it uses.
3. Raise the free chlorine. Superchlorination is the shock-treatment step in the algae protocol published by National Pool Industry News, which states that a black-algae protocol may need dosing at the top of the free chlorine band it publishes. Calculate the dose against the pool's actual water volume. The pools MFP Easy installs hold between 9,500 and 51,700 litres. A 4.5 m Verona holds 10,300 litres and a 7.5 m Bedarra holds 30,500 litres, so one instruction to shock the pool means a very different quantity of chemical in each shell.
4. Run the filter, then re-brush. Circulation carries the raised chlorine level across the whole shell, including the low-flow areas where the spots started, and the filter captures what the brushing has already lifted. Brush the treated areas again once the water has turned over.
5. Clean or backwash the filter afterwards. National Pool Industry News treats backwashing and filter cleaning as the step that takes algae biomass out of the filtration media, which is where the brushed-off algae ends up.
6. Repeat the cycle until the marks stop returning. Brush, hold the chlorine, run the filter, clean the filter and brush again, and keep running that cycle while any mark is still coming back to a spot that was cleared.

Is it safe to swim in a fibreglass pool with black spots?

Whether a fibreglass pool with black spots is safe to swim in is decided by the water, not by the marks on the wall.

Visible black-spot algae is a signal that the sanitiser has not been holding, and the water is what to fix before anyone swims. Test the free chlorine and the rest of the balance, correct what the readings show, then swim on the corrected water.

An osmotic blister is a shell condition, not a water condition. A blistered gelcoat does not change the chemistry of the water the shell holds, so the swimming decision on a blistered pool is made on the same readings as any other day: free chlorine, pH and the rest of the balance.

The two marks also send an owner to different places. Black-spot algae sends you to the test kit and the brush, and a blister sends you to an assessment of the shell.

What is osmosis in a fibreglass pool?

Osmosis in a fibreglass pool is water crossing the gelcoat and reaching the laminate underneath. A gelcoat is a coating over the laminate, and water molecules are small enough to pass through it. Once they are in the laminate they react with the polyester resin that binds it. The products of that reaction open small cavities, and those cavities swell outward, lifting the gelcoat off the layers beneath it into a blister.

Two causes underlie osmotic blistering.

The first is how the shell was laid up. The grade of the resin, the gelcoat and the laminate sets how much resistance the shell has to water crossing it, and a fibreglass mat that was not properly wetted out during lay-up leaves the laminate able to take water up.

The second is water reaching the laminate through the gelcoat.

The dark residue that leaches out of a blister is what an owner sees as a black spot, and that is why the two conditions get confused in the first place. A dark mark produced by osmosis is a by-product of the laminate, not an organism on the surface, and no amount of chlorine touches it.

MFP Easy installs Aqua Technics pool shells. Osmosis is a condition of older shells and of shells built to older specifications by other manufacturers, not something the Aqua Technics range MFP Easy installs develops.

What are the bubbles, lumps and bumps on a fibreglass pool wall?

The bubbles, lumps and bumps on a fibreglass pool wall are all the same condition: water sitting in the laminate behind the gelcoat. Owners name it by whatever they can see or feel, and the words change with the size and the stage of the blister.

• Bubbles on the wall. A bubble is gelcoat lifted clear of the laminate by a cavity that has filled and swelled behind it.
• Lumps and bumps under the surface. A lump is that same lifted gelcoat while the raised area is still broad and low.
• Soft raised patches. A patch that gives slightly under a thumb has liquid behind the gelcoat where solid laminate should be.
• A mark that looks like a stain but stands proud of the surface. A stain sits flat in or on the gelcoat. A raised mark is a blister carrying the dark residue that leaches out of the laminate.

Bubbles, lumps, soft patches and raised marks are not four problems. They are one problem at four sizes and stages, and all four call for the same response: an assessment of the shell.

What happens to a fibreglass pool once osmosis has started?

Once osmosis has started in a fibreglass pool, the blistering does not reverse. Nothing an owner puts in the water takes water back out of the laminate, because the affected material sits behind the gelcoat and nothing dosed into the pool water reaches it. Blistering does not hold still either. A laminate that is taking water keeps producing blisters, so the count rises over the years while the blisters already there stay where they are.

Products are sold to bleach the visible mark, and bleaching is all they do. Treating the appearance of a blister leaves the water in the laminate that produced it, so the mark returns as the laminate keeps leaching. A treatment that has to be repeated on the same spot is cosmetic work on a shell whose laminate is still wet.

Repairing a blistered shell is a resurfacing job, and it is specialist work rather than an owner task. A resurface is priced per pool after an inspection, because the extent, spread and depth of the blistering differ from one shell to the next. Having the shell assessed comes first: whether a shell is worth repairing at all, and whether it is ever emptied to do the work, is a decision for a builder who has looked at it.

How do you stop black spot and blistering coming back?

Black spot and osmotic blistering are prevented by two different routines, because they have two different causes. Four habits prevent black-spot algae:

• Hold the sanitiser rather than chasing it back. Black-spot algae establishes in the window where the free chlorine sits low, so the level that decides the outcome is the one the pool holds week to week.
• Keep the water moving. Circulation decides whether the shaded, low-flow parts of the shell are turning over or standing still. A dead spot in the flow becomes a spot on the wall.
• Brush the low-flow areas as part of the normal clean. A brush lifts algae off the gelcoat and exposes what is left to the chlorine in the water, so the shaded corners, the steps and the area under the returns are brushed on the same schedule as the rest of the shell.
• Treat a storm as a chemistry event. Rain carries organic matter into the pool, so a storm is followed by a test and a correction dose.

Two habits keep water out of the laminate:

• Keep the water balanced. Test and correct the water balance on the same routine that holds the sanitiser.
• Keep the pool full. An empty fibreglass shell is the risk, not a full one.

On a new pool the algae routine is started as part of the build. The build schedule carries an algaecide dose before handover, the filtration is built and the pool commissioned and started up as part of the install, handover is done with the customer present, and two service visits are scheduled after handover.

FAQs

Will chlorine remove black spots?

Chlorine at normal maintenance levels will not clear established black spot, which is why the treatment pairs brushing with a raised chlorine level. Michigan State University Extension's pool pest-management manual states that brushing scores the algal cell's outer wall and exposes it to the chlorine that kills it, so chlorine that never gets past that outer wall does not finish the job.

Chlorine does nothing at all to an osmotic blister. A blister is water in the laminate behind the gelcoat, and chlorine in the pool water never reaches it.

What chlorine level kills black algae?

National Pool Industry News sets its shock-treatment level at 10–30 ppm of free chlorine, with the band varying by which algae is being treated, and says a black-algae protocol may need a dose towards the top of that band. The figure describes a pool that is still full, and it is one publication's number, not an agreed industry standard.

Dose it against the pool's measured water volume, and keep brushing while the level is raised.

Is black spot remover just chlorine?

No. The products sold as black-spot removers are not one chemistry. Some are chlorine-based and act as a concentrated dose against the algae. Some are acid blends aimed at stains, which act on metal marks. One class is aimed at bleaching the dark marks osmosis leaves, which treats the appearance of a blister and is not an algaecide at all.

Buying the wrong one is the usual reason a treatment does nothing. An acid stain product will not kill algae, and a bleaching product will not take water out of a laminate.

How long does it take to remove black algae from a pool?

Removing black algae from a pool takes a repeated cycle of brushing and raised chlorine, and no independent source publishes a hold time or a brushing cadence for it.

Brush, hold the raised chlorine, run the filter, clean the filter and brush again. The treatment is done when a full cycle passes and no mark comes back in the same place.

What is the average lifespan of a fibreglass pool?

The lifespan question about a fibreglass pool is a question about the shell's interior surface, not about the structure. The shell is a structural laminate, and the gelcoat surface over it is the part that blisters, stains and marks.

MFP Easy installs Aqua Technics pool shells, and Aqua Technics shells carry the Lifetime Interior Surface Guarantee, conditions apply.