my 2 cents:
try root tabs. if your are doing EI water column dosing, then you have no nutrient problems in the water column. but you may see better growth (particularly from well rooted plants) if you use root tabs. i like root medic, but there are others (i have used flourish root tabs as well, and they arent bad).
i assume you have pressurized CO2?
Dumb question but, if you got a high CEC like flourite black, can't you just inject the diluted fertilizer into the substrate?