Hmmm. I'm not sure how the pH affects them. Mine is stable at 7.6. What's yours at? I heard tampering with your pH isn't so good though so I would leave it alone. What's the temp at? Could you show a picture of the whole tank? Just curious...
Oops. Didn't register that the pic was there. Sorry . And yes, the cories like the dark. That's why you should have plenty of shade (plants and such.) My old cories don't care much anymore about day and night but the fry get really active in the evening.