With a CYA level of 90, the MINIMUM chlorine level is 5, maximum of 10, according to the Best Guess chart. The shock level is 20+ ppm. With algae in your pool, It is recommended that you get your FC to shock level until the pool is clear, the CC is less than 1 and you maintain FC overnight. Just brushing is not enough, though it is essential. Raise you FC using bleach, not your SWCG.