Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I think the best time is AM, but not so closely linked to lights on or not. Just that the higher dose is available to the plants through the day, and by nightfall (lights out) most of it is gone, whether the plants used it or it broke down.
If you dose toward the end of the day you lose more of it to breaking down overnight, and the plants do not get the higher levels through the day.
Either way, a consistent dosing schedule is better than random, so if you are better at dosing in the evening or mid day, and tend to be irregular in the AM, then the mid day or evening would be better.