What FC (free chlorine) level is necessary to maintain a sanitized pool is entirely dependent on the current CYA (stabilizer) level. The more stabilizer present, the higher FC level that is needed. CYA is needed to protect FC from UV light but too much is a bad thing. For example, if you have 30 ppm CYA, you MUST maintain at least 2 ppm FC or greater at all times. If your CYA is at 60 ppm, you must maintain at least 5 ppm FC or greater at all times. This is for normal, daily sanitation. Minimum FC = 7.5% of CYA level. Indoor pools, it's not necessary to use CYA, but low amounts (10-30 ppm) can help buffer the harshness of chlorine and allow for easier maintenance of FC levels. Also, to clear a pool of algae, how high you need to raise the FC to fully eliminate algae depends on CYA level. Many times it takes longer than 24 hours of maintaining this higher FC level to clear the algae depending on how much algae, filtration capability... etc. For example, if your CYA is at 60 ppm, you'd need to elevate the FC to 24 ppm and keep it there until the algae is gone. "Shock" level FC is 40% of CYA level.