All diseases and symptoms are "treatable" but not all are "curable" and i think that you may mean "curable."
Fluid on the lung is an all embracing term, it could mean fluid in the lung tissue itself in which case the treatment is based on the cause.....pneumonia, heart failure and so forth, or more commonly it can be between the two coverings of the lung ( pleura) in which case it can be drained following the insertion of a tube, through the chest wall and into a container.
The two main types pulmonary oedema as directed by the link above and pleural effusion have different causes and require differing treatments.......but they can ALL be treated, but not all cured.
Is that what you wanted to know?