RATIONALE: Drugs used in chemotherapy, such as fludarabine and pixantrone, work in different ways to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing. Monoclonal antibodies, such as rituximab, can block cancer growth in different ways. Some block the ability of cancer cells to grow and spread. Others find cancer cells and help kill them or carry cancer-killing substances to them. It is not yet known whether giving fludarabine together with rituximab is more effective with or without pixantrone in treating indolent non-Hodgkin lymphoma.