If your CYA result is accurate your normal FC target is 4-8. Typically we recommend keeping FC at the higher end of the target range. You never want to let the FC drop out of target range. Your FC shock level is 16. When you start the shock process you want to raise the FC to 16 and keep it there until you pass the overnight test. That may mean several additions of liquid chlorine several times a day for a couple of days until the FC begins to hold.
You can use a skimmer sock on the skimmer basket to help catch some of the finer debris in your pool. You mentioned that debris was re-entering the pool through the return. Have you checked the multi port valve to see if it is functioning correctly? You can also check the gasket and extend the amount of time you rinse the sand after backwashing to see if that helps.
The debris in picture 3 looks like algae to me. Does it puff into a fine cloud when disturbed? That is typical of algae debris. Metals usually deposit on the surface of the pool as stains and not as debris.
What are you using to shock the pool now? What has been your usual source of chlorine?
You can use a skimmer sock on the skimmer basket to help catch some of the finer debris in your pool. You mentioned that debris was re-entering the pool through the return. Have you checked the multi port valve to see if it is functioning correctly? You can also check the gasket and extend the amount of time you rinse the sand after backwashing to see if that helps.
The debris in picture 3 looks like algae to me. Does it puff into a fine cloud when disturbed? That is typical of algae debris. Metals usually deposit on the surface of the pool as stains and not as debris.
What are you using to shock the pool now? What has been your usual source of chlorine?