Ecocomplete is a substrate that holds on to nutrients put into the tank, it hasn't any itself. The long list of nutrients listed on the bag are what the rocks are made of and aren't available for the plants, use the tabs even with the EC.
A quick google shows the double bright having about the same output as a single 18" bulb, is the tank wide enough to put both on the tank? The 18" bulb might put out 20 units of PAR at 24" off the substrate and the double bright might do the same which would put the light at closer to 40 units of PAR which is medium light.
Yes I have both the Marineland double bright and the t8 bulb in the tank together. I'll upgrade the t8 to a T5HO if I can find the right length. Then hopefully light will be good and I can work in the substrate and nutrients.