20 gal tank - good mix of community fish ?

Since the Corydoras catfish are doing fine, and you are getting good readings on the water, it should only mean one thing if the Cardinal Tetra are not active -  They have too few in number.   So they are stressed.   5~6 is the minimal requirement for a school.    The more the better.   When I added over a dozen of them to a new tank, usually they are immediately happy to be in the new aquarium.    A bigger group will bring out their natural behaviors.   It is the same for all school fish.  

As for the plants, they are clearly not doing well.   They need nutrients (not only nitrate, but they also need various minerals), adequate lights, and CO2 to thrive.   They usually do not do well with regular gravel.  It is why good substrate rich in minerals is so important for a planted aquarium.    There are some commercially available aquarium fertilizers you might want to check into.   I have used Flourish Tabs in the past.  Not sure how effective it was since I already had mineral rich Eco-complete substrate.