to lower down the ph you can use....peat from brand like sera,fluval(hagen),JBL,Azoo,tetra and etc
but better to have a big bucket,produce the lower PH water there....only you put the water in your tank,coz some of this peat can drop the ph until 4.0-4.5ph
what i was thinking its your co2 diffuser,one of my fren have high lighting system with good filteration also, 2-3 bubbles per sec,for a 3 ft tank,he use those diffuser that make of glass,which is tranparent,the diffuser will make the co2 bubble into a finer bubbles and release out from the gas stone.

but problem is,the co2 is only going upward,but not touring around the tank,so he have changed to those greenish diffuser from Up brand,function by some bio ball spinning around in the cilinder,of coz you will need a powerhead to make the thing move.....so the flow will take the co2 travel around the tank....after few days....riccia and some plants start to bubbling.

hope this will help