Commercially grown plants are cultivated “emersed” rather than “submerged”. This way the plants can easily get their carbon requirement from atmospheric
CO2. They can be grown quicker which makes the operation much more commercially viable. You take that plant, put it in your aquarium, cut off its source of
CO2 and the plant goes into survival mode. It starts to use up its stored carbon and the leafs melt. You may lose all your original growth to melt but new leafs will have a structure more suited to its new environment and get its carbon from the water. Plant melt is a normal stage in aquarium plant growth. To judge the health of a plant look for new growth rather than what might be happening to the original growth, and judge it over extended periods of time.
Remove dead and dying growth so the plant can concentrate its resources on new growth rather than trying to support dying leafs.
Plants need a level of nitrogen and in aquariums they typically get this from nitrate. From one of your previous posts your plan was to stock lightly and gradually increase the number of fish. So you may be low on this essential nutrient, but as you increase the number of fish your nitrate level should increase.
Your plants should benefit from an all in one liquid fertiliser and root tabs in the substrate around the plant.