What Causes Fishy Odor Vaginal Discharge?

Suffering for past 3-4 years with fishy odour vaginal dishcharge, have had hvs and sexual screening everything negative all the time so no problem there. Also have had scan of pelvis abdo no cysts or anything. This is affecting my day to day life the discharge is not heavy but I am conscious of the foul odour and have tried anti biotics many times which temporarily addresses the problem and then it returns again. Am so fed up what is this?