Sand filters can only remove particles down to a certain size. The smallest particles that are not caught by the sand filter can form a cloud in the water, which gives the pool water a dull and dull sheen. To capture the smallest particles, the flocculant is added, which binds the smallest particles together into larger units that can now be captured by the pool's sand filter.
Cloudiness in the pool is very often due to the water's pH value being or having been too high for a period or organic pollution. Test whether the pH value is 7.2 and, if necessary, clean the water with a chlorine shock.