Hi James,
Welcome back to the hobby With 12 discus(2") in a 40 gallon with gravel, you need to do water changes every day 80%. When you buy discus at 2" size you need to be prepared to maximum water changes and more feedings. I would increase the feedings to 6 times a day. You can include beefheart(not mandatory) and freeze dried black worms - both are very high in protein. It's good to remove the uneaten food, with daily water changes you can siphon that out.
Regarding the ph I had so many questions myself when I started the hobby. Biggest Key is "Stable PH" I have all my tanks running at 7.8ph and at my previous residence I raised Discus with ph 8.2. Do not bother using PH down chemicals. PH swings is way more dangerous than having a stable ph.
- Sanjay