What is the things that will leads to poor memory and how can i improve it? is fish oil really helps?

Memory troubles: Can be caused by a number of things. First, get to your family doctor for a work up to rule out any medical issues. Then a neuropsych eval with testing would help to identify the exact problem. A treatment plan for you could then be developed to help with your specific problem. And yes, fish oil may be helpful.

Underlying cause: Vitamin d or B12 deficiency, hormonal imbalance (low testosterone, or thyroid abnormalities), electrolyte imbalance, sleep deprivation, stroke/meningitis, lyme disease, central syphilis, aids, autoimmune disease, medications, alcohol or substance abuse, prolonged stress, diabetes, hypertension. Treat underlying cause. Active exercise, optimal sleep, avoiding alcohol/substance, cognitive challenge.