Your plant is Alocasia gageana, not Colocasia. it is showing severe deficiency of either magnesium or manganese, as it is the older leaves that are chlorotic. If the newest leaves were chlorotic, it would be an iron deficiency. Epson salts is good for magnesium, but for manganese you'll need to go to an ag or nursery supply house to get your remedy.
LariAnn
