The best time to test your parameters is the same time every day
What exact time that is shouldn't matter a great deal but you should consider your system's setup. If you don't have a sump with a counter light schedule then your Ph will fluctuate with your light schedule due to the change in O2
levels. If you have a sump that is lit when your main is unlit then your Ph and O2
levels should stay more consistant. Other than that there shouldn't be a great deal of change in water parameteres throughout the day.