Sand bed plugged with mineral deposits.
Partially closed valve.
Chemically clean filter.
Open valve or remove obstruction in return line.
Pool chemistry not adequate to inhibit algae growth.
Plugged sand bed
Manually remove top 1″ surface of sand bed. Replace with new sand and chemically clean entire sand bed.
Restrict flow to capacity of filter.
Improper sand size
Air strainer is damaged or missing
Change to proper sand.
Replace air strainer.
Dirt or contamination
Shut off pump, relieve tank pressure, open air bleeder, remove closure & clean all sealing surfaces. Reassemble closure carefully.
Same as above except replace damaged O-Ring, closure, tank, or any combination of parts as required.