I think you probably have low-medium light, so CO2 isn't necessary. If you use plant tabs poked down into the substrate I doubt that you need to dose the water, although it won't hurt to do so, providing you dose lightly and only about weekly. With no CO2 or Excel you shouldn't do regular big water changes. Just replace the evaporation losses.
Even though CO2 is probably unnecessary, adding CO2 will give you faster plant growth. Then you could use an EI type of fertilizing method. Or you could use Excel for carbon, and again use an EI type of fertilizing method.
I suggest reducing the lights on time to 8 hours.