First off, balance your water, paying particular attention to pH. Secondly, check your filter system and clean if necessary. Turn on automatic cleaners to help stir things up.
For suspended green algae, shock the pool. Thoroughly brush off any algae that may be clinging to the pool surfaces.
Add 1kg per 50m3 (11,000 gals) of the Fi-Clor Superfast shock Granules. Broadcast the product evenly over a wide area in the deepest part of the pool. It should only be used when bathing has ceased. For outdoor use, avoid facing the wind when dosing. For heavy algae contamination, double this dose rate may be required. Run the filter for 24 hours, then backwash it and vacuum the pool thoroughly. Any remaining haziness in the water should be removed using one of our water clarifiers.
To help prevent a recurrence, ensure that free chlorine levels never fall below 1mg/l (ppm) for unstabilised or 2mg/l (ppm) for stabilised chlorine pools. Also dose the pool with a long life algaecide.
After the chlorine level has come down below 5 ppm, add an algaecide and brush the pool again. When it all settles, vacuum the pool (to waste, if possible). Check and re-balance the pool water if necessary.
For algae which is not suspended, but only clinging to the walls, follow the same advice above, first shock with brushing, then add an the Hot Tub and Swim Spa Summer long product, or a Spring Long-life Algaecide Hot Tub and Swim Spa. Add 5 litres of algaecide per 50m3 of pool water
After doing all these treatments, test your water and make sure all the pH, Chlorine, Alkalinity is where at the right level, otherwise, use the adapted chemicals to balance your water.