In lieu of a clarifier additive, one method of flocking a sand filter is to disolve granulated Shock in a plastic pail of water and pour into the skimmer and filter the water for 48 hrs. What it does is tighten the space between sand particles so smaller particles can't escape. As that happens, the filter's efficiency increases as more smaller particles are trapped. Sort of a cumulative effect.
One better method is simply to not backwash the filter so much - maybe once per week or when the pressure is above 20-25 psi.
Either way, chlorine should NEVER be disolved in a house or an enclosed environment.
How often should sand be changed. I have heard that if filter is working properly it should be every 5 years as it works more efficient when it has dirt in it.
I have another friend who changes it yearly.
My pressure never has gone over 15 psi is that normal. I backwash maybe once or twice per month.