Agreed. I inject around 8bps in a 21 long tank, which results in a 1.2 pH drop. Even with that injection rate, it takes over an hour to hit 1.0 pH drop and I start injection 2 hours beforehand. This graph is from my tank (about 6 weeks ago), taken using a pH probe and reef-pi. If you don't start beforehand, you're giving plants light and nutrients, without the corresponding CO2 for a significant amount of the photoperiod.