Try dosing macro / macro ferts directly into the water column. Just excel will not give the plants everything they need.
Dosing with root tabs helped you out but most plants feed mostly from the water column. Truth is that almost all stem plants and such don't even need to be buried in the substrate. Heavy root feeders like swords, crypts, etc. will greatly benefit from substrate fertilizers, yes, but you need to dose the water column to make your plants happy.
Note that the #s in that thread are for high tech / high light tanks. I dose 1/2 EI in my low tech with good results. I also use the planted + on each of my low techs so I can confirm that your lighting will probably work. Also if you do this go slowwww because if you put in too much too fast algae can take over.
What kinds of plants do you have in your tanks? How long do you run your lights?