your guess is wrong, the pictures were taken in the middle of a water change hence the low levelAt a quick guess, the issue is your water level.
Water from your filter will force floating plants under water because of the height of the 'waterfall'.
This will submerge floating plants, keep them from the CO2 they need, and they die.
Having a barrier to keep them from this water may help, or just add water to the top of the tank
it is hard to pick a specific plant when LFSs staff don't know what they sell. I don't remember which one but i do remember i once bought a plant because LFS said it was one i wanted only to later find out it wasn'tHere is a sword for small tanks. I started mine in a 50 gal highlight about 18+ years ago. It made babies. I moved one to a 20L then upgraded the tanks to a 29. I still have it. The 50 was repurposed abput 9 or 10 years ago. I know nothing about this site, so you may want to search the plant on Google to find other places to get it.
I still appreciate your help, and definitely will be on the look out for this amazon campacta