I think, and this is usually when I am wrong, it isn't really the high pH alone that kills fish, it is the ammonia issues that follow. If ammonia is present at sufficient levels, when the pH rises, NH3 is produced, which is highly toxic. Your ammonia levels may not reach that level, plus it is usually in the 9 range of pH when that begins to occur.
There is a lot going on here and I wish I could be more help. No matter what your pH level is, what you need to be more concerned with is your ammonia levels and phosphorus.
You don't really need 3am, although it is optimum, you just need well after sunset to compare the readings to mid day.
