I just replaced my Eheim peat and did measurements.
20gal long, PH without peat 7.0.After adding 1/2 men fist size peat in to the canister PH drop to 6.1 .It will stay 6.1-6.2 for 5-7 days then ph will go to 6.6-6.7 for 3-4 and will stay there for 12-15 days. Then back 6.9-7 after 2-3 more days. One box Eheim peat $9 will last 10 replacements-9-11 months.
I'm using RO PH6.5 after one week in a bucket it drop to 4.8-5.2.problem is my Eheim 2215 filled up with Eheim Bio media 4-5L of it. Looks like it buffer water to PH7
Substrate ADA Amazonia old -2 years old-no buffering at all.
In other tank with 1 year old ADA PH stay at 6.7 .