I don’t really know why as it’s an established planted tank. Maybe do a 50% water change, trim the plants, and maybe add some fertilizer for the plants/nitrates. Possible when cleaning the mechanical filtration you used tap water killing the bacteria, causing everything to be zero and the new colony of bacteria needs to grow? I have no evidence to back this up, but I feel like the bacteria is hungry and needs to be fed some nitrates lol.