As a rule, old leaves do not rejuvenate, as plants channel their energy towards new growth.This means that a yellowing leaf, or one already damaged with algae, will not become green again, no matter what you provide. Look out instead for how the new leaves are doing: with good nutrition, they should be richer in color, be in better form and remain algae-free. For stem plants, you must replant the healthy new portions grown under APT, and discard the old stem and roots after the new portion has grown tall.