On a small nano tank the wpg rule doesnt really apply. Excel is basically a Co2 substitue, what your plants are missing are nutriants. you need to start dosing ferts to get the best out of your plants. basically your giving them the right conditions to grow fast but nothing to keep them healthy as they grow. ferts are like food to them.
Are you using Seachem Flourish or just Excel? flourish excel are 2 different products...
If your not I would start using Seachem Flourish every other day and also Seachem Iron on alternating days. And then just keep using Excel like you have been. if you have been using flourish you might want to try to increase the dose.
What substrate do you have in the tank? is it new? might want to try some root ferts in the gravel if its a new substrate.
Also what plants are you trying to grow, some just dont do well without real co2 injection, root fertilization, and high light....other need more iron in the water. theirs alot of factors involved for plants. these are just a few things off the top of my head.