Go to Dr. Try elastic support either with knee elastic brace or wrapping with ace bandages, but most likely you need aspiration of the fluid with a needle/ syringe done by a rheumatologist, an orthopode ( especially if the knee was operated) or a pcp (primary care physician )that feels comfortable doing it. Apply cold if recent injury, warm if chronic. Also r.I.C.E. (rest, immobilization, cold, elevation).
Ice, bike, NSAID . Joint effusion or excess fluid in the knee is usually from a flare of arthritis or trauma but could also be from an acute injury to the meniscus or acl. Moving the knee with low or non- impact exercises ( like stationary bike) will help put ice behind and in front of the knee after exercise or work. Take an antiinflammatory like Ibuprofen for 5-7 days ( with food).