Pool Tips: Dealing with black spot algae

27 December 2021

 by Angus Donald

Few things are more pleasing to the eye than your perfectly presented pool and thus the arrival of Black Spot Algae is perhaps one of the most annoying and hardy of pool care troubles. Not to worry, a solution is close at hand.

Black Spot Algae can be present in your pool as either small black spots (often starting on the steps or in the tile-band), or as larger areas of dark patches on the floor or wall of the pool.  These dark patches are an indication that the Black Spot is growing in the pool, however, be aware that copper staining can look similar.

To confirm the dark patches are Black Spot Algae, and not copper stains, try to scratch some of the algae off the surface, or sprinkle some stabilised granular chlorine onto a small area.   If the Black Spot comes off, or is bleached by the chlorine, you can be confident it is Black Spot.   On the other hand, if the chlorine and scratching has no effect, and the marks appear to be a discolouration rather than a growth, then you are probably dealing with a stain (and that's a whole other Tip!).

Black Spot Algae grows in the concrete walls of your pool, or in the render in your pebble-crete surface, or in the grout between your tiles.  Black Spot will not grow on fibreglass or vinyl.

Check the water chemistry and the function of all filtration equipment to ascertain why the pool was infested with Black Spot to start with, otherwise, you might treat it and it will very likely return.

Treatment
- Assess the severity of the algae infestation.   Less than 20 spots would be considered a light infestation, more than 20 is heavy.
- Test the pool water.   To treat Black Spot, the pH should be lowered to between 7.0 and 7.2 and the copper levels need to remain below 0.6ppm.   This will only be a problem if the pool has had a recent algae treatment, or is sanitised using an Ionizer (do not use copper based algaecide in Ionised Pools).
- Ensure filter, baskets and salt cell are clean.  Remove your automatic cleaner and vacuum plate (if installed).
- Add acid as required to lower the pH (use acid demand test).
- Give the Black Spot a brush with your pool broom or an algae brush to scrape the surface of the Black Spot.
- Add stabilised granular chlorine, sprinkling it directly onto the Black Spot where possible (you may need to turn the pump off in order to get the water still enough to land the chlorine on the algae).   Use 1kg stabilised chlorine for light infestations, 2kg for heavy (based on a 50,000L pool).  Allow the chlorine to sit on the Black Spot for as long as possible (10-15 mins).
- Add 1L of Powercide 4 Algaecide Mix the algaecide in a bucket of pool water, and then spread it around pool whilst the pump  is running (this dose is for a pool of approximately 50,000L - Adjust dosage for the volume of the pool to be treated).
- Brush the pool, paying particular attention to the Black Spot.   Black Spot gets a waxy coating, and this must be scratched to allow the algaecide to work.   For this reason, it is important the Black Spot is brushed regularly until it is all gone (this can take up to 2-3 weeks).   If a small amount of Black Spot is left in the pool without brushing, this will start to spread again as soon as the algaecide has lost its potency.
- After the initial treatment, you will need to vacuum your pool, or run your automatic cleaner, as normal, and thereafter clean your filter on a regular basis.
- Remember to brush the Black Spot every day until it is gone, and re-balance the water (bring a water sample into one of our shops).

Very Important:
WHEN USING A COPPER BASED ALGAECIDE DO NOT ADD BUFFER OR CALCIUM TO THE POOL WATER
In normal circumstances these chemicals are essential for your pool, however, they will increase the pH of the pool water, and if excess copper is present, the high pH can cause the copper to "plate out" and stain the pool a nasty black colour.