I can’t keep Nothobranchius Killifish alive; they always die within a month or two after I purchase them. My water parameters are perfect Nitrites, Nitrates and Ammonia are 0 and the PH is balanced at 6.5, What am I doing wrong?

The Nothobranchius family are known as annuals and their lifespan rarely succeeds 10 months. So, the reason your Killifish are dying is likely due to the simple fact that they were already at the end of their lifespan when you purchased them. 

If you want to pursue keeping Nothobranchius we always recommend starting from eggs and growing them out, not only do you get to enjoy the Nothobranchius as long as possible, but you get to experience their miraculous life cycle.