Plant melt is normal when introducing new plants to an aquarium.
Commercially sold plants are cultuvated immersed rather than submerged in water. This way they have ready access to CO2
from the atmosphere and will grow much quicker and be more commercially viable. The downside is that the plant then has to adapt to new conditions when put in an aquarium, leafs that have had ready access to CO2
will struggle without it and may die off. Its possible to lose all your original growth, but new growth should be tolerant to being submerged. A week isnt really long enough to judge new plants.
Remove any dead or dying leafs so the plant can concentrate its efforts on new growth.
Are you seeing any new growth yet?