I would buy the pH argument if the behavior of the other fish had changed, but it hasn't. And I would buy the temperature argument if the temperature had changed, but it has been consistently at 76 ever since I set it up and the juliis were happy for a couple months at that temp.
I'm more inclined to start watching how much food they're getting, although I tend to think they're getting plenty because I feed everyone a good amount and I do see food get to the bottom of the tank, plus I feed a variety of foods -- daphnia, flakes, bloodworms, shrimp, sinking pellets, peas, spinach, etc.
Wasserpest, when you said, "This is very apparent with Cories, they tend to go up and down the glass a lot, but later they settle down", how soon is "later"? A few days? A few weeks? Because mine were pretty active for the first couple months but then a few weeks ago really started slowing down.