Fluid on knee. The best doctor would be an orthopedist but some general practitioners can do and er doctors.
Orthopedist. Most orthopedists can drain the fluid off. However, many effusions (fluid within knee) return after draining. Your surgeon should also find the cause of the fluid to begin with. Some causes of fluid may include: meniscus tears, acl tears, new cartilage tears, infections, and a variety of inflammatory arthridities.