Ok, i think i went through this last year.
My ro water is about 6.8-6.9 and i want it a bit lower so i try experiment with peat.
First i bought some of the peat sticks ( the one they feed koi in pond), put them in mesh bag and leave them in filter.
Result was not good, after a while it turn bad, smell bad, and did bad thing to my water
Next, i tried borneowild humic. My water was 6.4-6.6.....It's very good but also expensive. I have 3 tanks and not about to spent $60 for them every 5-6 month.
Then i found the solution, i found sera super peat. Bought 2 boxes on flea bay for less than $20 shipped.
I put them in a small mesh bag and leave them in my fluval305. Water measure at 6.4-6.6 just like humic.
Have not change it yet for a month now but it's cheaper that i probably change them often.
Very happy with it and a simple solution for me.