PH IN TANK TO LOW

Question
MY PH IN MY TANK IS ABOUT 6.4 AND I AM TRYING TO KEEP 3 OSCARS IN IT AND TWO HAVE DIED WHAT CAN I DO TO MAKE IT GO UP BESIDES CHEMICALS

Answer
Chad-
To directly answer your question, you can get substrate that is designed to release particles into the water that make it harder.  Typically this is used in African cichlid setups, but can work well in any area where PH is too low.  On that note, Oscars have been known to be extremely hardy animals, and tolerate a huge variety of PH.  How long has the tank been running?  I'd need that to be able to better assist you.  Good luck with the Oscars.  Post back with some more information on the tank readings (nitrates, nitrites, ammonia)