What you are probably seeing is normal with new plants and is called melt.
Your seller is correct that plants are cultivated emersed (not fully underwater). Emersed plants have ready access to CO2 from the atmosphere, and can therefore grow quicker and make commercial aquatic plant growing more commercially viable.
What that means for your plant though is that when you put it in your aquarium fully underwater, that CO2 is largely cut off, and the leaf structure cant cope with the change and melts. This isnt to say you need to be injecting CO2 though. New growth will be more tolerant to its environment, and get CO2 from the water through a different mechanism to what the original growth did from the atmosphere.
This can all take time though. Judging plant growth over a few weeks isnt giving them long enough to transition from emersed to submerged. Are you seeing any new growth?
The thing to do is remove any leafs that are dead or dying so the plant isnt wasting resources trying to maintain old growth and can focus it on new healthy growth. You may over time lose all the original growth but as long as its replaced with new growth thats not an issue.
You also should keep low demand plants unless you really want to be getting into specialist lighting and injected CO2.
Ive used the tropica fertiliser before with good results. Rooted plants could also benefit from root tabs in the substrate.