Algaecide for pools is a preventer and a helper, not a cure. Chlorine is what kills algae; algaecide makes it harder for algae to take hold again once the chlorine has done its job. Pour it into a green pool on its own and you’ll mostly waste money.
Used at the right time, though, a good algaecide earns its place, especially in spring when Cape Town pools warm up and the first blooms appear. This guide covers the types on the shelf, when to add them, the stubborn black and mustard varieties, and the mistakes that leave you with foam or stained marbelite. If your pool is already a swamp, skip ahead to our green pool recovery service.
What algaecide for pools actually does
Algae spores blow into every pool, all the time. They arrive on wind, rain, leaves and swimsuits. Whether they grow depends on three things: sunlight, warm water and weak sanitiser. Chlorine at the right level with balanced pH kills spores before they multiply.
Algaecide works in the gaps. It attacks algae cells in a different way from chlorine, so it gives you a second line of defence when chlorine dips, for example after a hot weekend of heavy swimming or a few days without the pump. It does not replace chlorine, and it won’t fix a filter that isn’t keeping up.
Algae in pool water: why it comes back
If you find algae in pool water every few weeks, the cause is nearly always one of these, and algaecide won’t fix any of them:
- Free chlorine regularly dropping to zero between doses.
- pH drifting high, which makes chlorine much less effective.
- Too much stabiliser, which slows chlorine down.
- Poor circulation, with dead spots behind steps and ladders.
- A tired filter, or one that’s run for too few hours a day.
We cover each of these in more depth in our guide to a green pool that keeps coming back. Fix the cause first, then use algaecide as insurance.
Types of pool algaecide
Read the active ingredient on the label. It tells you far more than the brand name or the colour of the bottle.
| Type | Label name | Strengths | Watch out for |
|---|---|---|---|
| Quat | Quaternary ammonium, often “alkyl dimethyl benzyl ammonium chloride” | Cheap, widely available, no metal | Foams, especially lower-strength products |
| Polyquat | Polyquaternary ammonium, often “polyquat 60” | Non-foaming, non-staining, good for prevention | Costs more per litre |
| Copper algaecide | Copper sulphate or chelated copper | Strong on stubborn types | Staining and green-tinted hair, especially at high pH |
For most home pools, a polyquat product is the safe all-rounder. It won’t foam, it carries no metal and it suits marbelite, fibreglass and painted pools.
Copper algaecide on marbelite pools
Copper algaecide works well, but copper can drop out of the water and leave blue-green or dark stains, especially when pH runs high or when chlorine levels are pushed hard. Porous marbelite surfaces are the most likely to mark, and those stains are hard to remove. If your pool has an older marbelite finish, we’d steer you to polyquat and keep copper for specific problems, at the label dose only.
When to use algaecide for pools
Timing is where most people go wrong. Here’s when it helps and when it doesn’t:
- Yes, as a weekly preventive dose. A small maintenance dose with balanced water keeps spores from settling in.
- Yes, after you’ve cleared a green pool. Once chlorine has killed the bloom and the water is clear, an initial algaecide dose helps stop it returning.
- Yes, in early spring. When the water warms up in September and October, algae wakes up too. A dose with your first proper spring clean is good timing.
- No, as the only treatment for green water. A green pool needs a chlorine shock first.
- No, straight after shocking. Very high chlorine breaks down many algaecides. Wait until free chlorine drops back towards the normal range, as the label directs.
Our post on how much shock to add to a pool covers that first step. Algaecide comes after.
Pool already green?
We shock, brush, vacuum and balance the water, then leave your pool with a simple plan to keep it clear. Send us a photo and tell us your pool size.
How to add algaecide for pools, step by step
Every product has its own dose, so the label is your guide. The routine around it looks like this:
- Test the water. Get pH into range and confirm chlorine is present but not at shock level.
- Clean the baskets and backwash the filter if the pressure is up.
- Brush the walls, steps and floor so spores are lifted into the water.
- With the pump running, measure the label dose for your pool’s litres and pour it slowly around the edge of the pool.
- Keep the pump running for at least a full day so it mixes through the whole system.
- Retest chlorine and pH the next day, and then stick to the label’s maintenance dose.
Wear gloves and eye protection, add the product to water rather than water onto the product, and never mix algaecide with chlorine or any other chemical in a bucket. Keep children and pets away until it has circulated.
Black algae and mustard algae: the stubborn types
Ordinary green algae is the easy one. Two other types need more effort.
Black algae
Black algae shows up as dark blue-black spots, often the size of a coin, on rough plaster and grout lines. It roots into the surface and grows a protective layer that shrugs off normal chlorine levels. Pools with older, rougher marbelite are most at risk because the spots have somewhere to hold on.
The fix is mechanical as well as chemical: scrub each spot hard to break the layer, keep chlorine at a raised level for days rather than hours, and repeat the brushing daily. An algaecide labelled for black algae helps after the chlorine phase. If spots keep returning in the same pits, the surface itself may need attention. A rough, porous finish is one of the reasons people choose marbelite resurfacing.
Mustard algae
Mustard algae is yellow or brownish and looks like pollen or fine sand in shaded corners. It brushes off easily and then comes straight back. It also hides on pool toys, brushes and cleaner hoses, so clean those during treatment. Brush, shock, keep chlorine up, then use an algaecide that lists mustard algae on the label.
Is it really algae? Brownish dust that doesn’t brush away may be fine dirt from the summer south-easter, and green-tinged water with no slime can be metals. A proper water test before treatment saves you buying the wrong product.
Pool algaecide mistakes that cause foam and stains
Most algaecide complaints we hear come from one of these:
- Foam. Cheap quat products overdosed in a pool with a waterfall or spa jets. Switch to polyquat and stick to the dose.
- Cloudiness. Too much algaecide, or algaecide added to a pool full of dead algae that the filter hasn’t caught yet. Our guide to a cloudy pool after shocking explains how to clear it.
- Stains. Copper products in a high-pH pool, or with a marbelite finish.
- No effect. Algaecide used instead of chlorine. It can’t keep up on its own.
- Blooms after rain. Winter rain dilutes chlorine and washes garden soil and phosphates into pools. Test after a downpour instead of waiting for the next routine dose.
Phosphate removers are another tool you’ll see. They take away a food source for algae, which can help a pool that keeps turning even with good chlorine. They don’t kill anything, so treat them as a support product, like algaecide.
At the time of writing (October 2026), 1 litre bottles of algaecide sell from around R145 at SA pool shops. That bottle only goes far if the chlorine and filtration are doing their part. Ask our team for a current quote if you’d prefer us to handle it.
Algaecide for pools: your questions
What does algaecide for pools do?
It stops algae from getting established and helps chlorine finish off stragglers. It is a preventer, not a replacement for chlorine. Keep chlorine and pH in range and use algaecide as backup.
Algaecide vs chlorine: which kills algae?
Chlorine kills algae. Algaecide helps prevent it from coming back. For a green pool, shock with chlorine first, clear the water, then add algaecide once chlorine has dropped back towards normal.
When should I add algaecide to my pool?
As a weekly maintenance dose, after clearing a green pool, and with your first spring clean when the water warms up. Don’t add it straight after a big shock, because high chlorine breaks many algaecides down.
Does algaecide make a pool cloudy?
It can, if it’s overdosed or if the water is full of dead algae the filter hasn’t removed yet. Run the pump, backwash when the pressure rises and give the filter a day or two. Persistent cloudiness usually points to the filter or the water balance.
What is the best algaecide for pools in South Africa?
For most home pools, a non-foaming polyquat algaecide is the safest all-rounder because it carries no copper and won’t stain marbelite. Use copper products only for specific problems and at the label dose. The best product still won’t work without proper chlorine and filtration.
If the algae keeps winning, our green pool clean-up team can get the water back to clear, and then regular pool maintenance keeps it there.
Get your pool back to blue
Tell us how green it is, how big it is and how long it’s been like that. We’ll quote before any work starts.
