It probably sounds like I'm talking in circles, but the idea is that the effectiveness of the dissolution is related to injection rate, timing, as well as flow rate and distribution patterns, so having to address 4 different variables can be confusing.
If there are no fish in the tank then injection rate increase is the easiest thing to change. Flow rate is next easiest, by pump addition or filter upgrade, or by media deletion, then, most difficult is to fix, the distribution pattern. I need to see a schematic or an actual image of your flow outlets so that I don't get confused.....