The phrase "reached a peak" implies that something has reached its highest point or peak level. Antonyms for this phrase could include "bottomed out," "hit rock bottom," "hit a low point," "plummeted," "declined," "sank," "dwindled," or "regressed." These words all suggest a decrease in height, level, or quality, rather than an increase. For example, if a company reached a peak in profits last year, this year they may have hit a low point or bottomed out. Alternatively, if a mountain had reached its peak, it may start to dwindle down in size over time.