Don't use any additives or buffers they can be be dangerous, I think you need an increase in maintaince and water changes. A ph from 6.0-8.0 is fine the important thing is that it is consistant, and changes are minimal. I think your low ph is from organic waste loading, here's a few questions:
Whats the ph of your tap water?
Whats your maintaince route now?
What the rest of your water parmameters ie: ammonia,Nitrite, Nitrate?
Filter media used?