This is why people shouldn't get "the minimum-sized tank." Within a year a problem arises: change the water three times a week, let the fish get sick, or trade it in. As though it's a video game rental and not a living being who was not created to amuse us.
Oscars can recover from HLLE with low nitrates and good nutrition. Boyd Vita Chem is recommended by many. (Soak pellets, veggie rounds, and freeze-dried krill with it. You don't need to pour it in the water.)
Adopting an Oscar is commendable. I salute you.