Driftwood will lower ph but may stain water. Just boil or soak it first to get rid of some of the tannins that will stain the water. Also WC's will keep the staining effect down.
MFK MEMBERS WILL PROBABLY JUMP ALL OVER ME FOR SAYING THIS. BUT WHEN I WAS BREEDING DISCUS I USE TO LOWER THE PH BY USING COMMON OLD HYDROCHLORIC ACID, ITS CHEAP HAS A GOOD SHELF LIFE AND IT WILL GIVE YOU INSTANT PH RESULTS, AND BEING LIQUID IT DISPERSES INTO THE WATER VERY EVENLY. DO NOT ADD IT TO THE TANK WITH FISH IN IT MAKE IT UP IN A SEPERATE CONTAINER TO THE DESIRED PH BEFORE ADDING TO THE MAIN TANK.
IF YOU HAVE PLANTS IN THE TANK CARBON DIOXIDE INJECTION WILL ALSO LOWER YOUR PH BY PRODUCING CARBONIC ACID. HOPE THIS HELPS......