How can you get rid of that fluid behind the ear drum?

Serous otitis: Anti-inflammatories and decongestants can get rid of this. Nasal sprays and sometimes antibiotics are helpful as well. See your PCP if these dont help.

Fluid behind the ear drum?: If truly have fluid behind ear drum, did this follow ear pain or infection or following a cold? Has hearing been affected +/or dizzy? Most common+ safe treatment is increase humidification (hot showers , vaporizer) + using nasal saline spray to open up sinuses/Eustachian tube . Might need to see Doctor if infection also, then needs antibiotic. Many benefit from short term of steroid to open