problems with plants ...

[hoolagal]

i have a 30 gallon freshwater tank with low light (standard canopy, standard bulb) ... the tank includes 4 dwarf puffers, 6 otos, and 11 amano shrimp ... my water parameters are (and have been consistently) ammonia 0, nitrites 0, nitrates between 10 and 20, and pH 7.2 ... for the first eight months my plants did really well until i added a plant that had black brush algae ... i dealt with that and the algae seems to be gone ... i did change my substrate from fine gravel to 3m colourquartz white sand, every tank has sand now ... it has been over a month, so i went out and bought a lot of new plants (all for low light, all classified as easy, and from a real aquatic plant service) ... it has been three weeks now and my water parameters are good, i just did the measurements above before typing this (i do weekly water changes from 25-50%) and weekly i add half a capful of flourish excel which is the correct amount for my size tank and my plants are not doing well ... in fact, one has melted completely away and two others have died completely ... some are still alive but not thriving, but in my other tanks, smaller tanks they are doing well, even in the 2.5 gallons !!! i don't know what the problem could be ... are there other water parameters i should be checking for in tanks with live plants ??? any suggestions ??? what am i doing wrong ??? where did my green thumb go

crypt balansae
Echinodorus 'Red Flame'
Lysimachia nummularia 'Aurea'
Eleocharis vivapara
some kind of sword plant
java fern
Hygrophyla polysperma 'Ceylon' (this one seems okay)
P. Stellatus (this one seems okay)
Ludwigia repens 'Rubin' (this one seems okay)
there are a few other i do not know the names of