If you have an air pump available try this - take about a quart of tank water and test the ph, then aerate the water heavily for 30 minutes or so and test the ph again. If your ph increases much after aeration then you know your have higher CO2 levels in your tank which are probably causing your low ph problem. Also keep in mind that ph varies over the course of the day so depending on when your testing you may not have any ph problem at all. My ph is at it's lowest in the morning before the lights come on.
Alot of people get into trouble when they start using too many additives. So a person starts adding buffers to raise the ph which also raise the alkalinity. Then they have problems maintaining calcium levels because their akalinity is way too high so they throw a bunch of products xy&z to fix that and on and on....great for LFS's like me but bad for your tank and your wallet.
I've found my tank does best when I don't muck with it too much. Once you get things under control try just using a balanced calcium additive (kalkwasser or a two part additive) and regular small (10% or so) water changes. Some people swear by daily iodine additions but the jury is still out on wether this is required or not IMO.