What hobbyists mean by melt
“Melt” is a description of leaves turning translucent, soft, or breaking down. It is not a single diagnosis. Changes in growing conditions, damage, poor planting, and unsuitable conditions can all lead to leaf loss.
Inspect where growth begins
Check the crown or rhizome. A firm base with healthy new shoots is more encouraging than the appearance of an old leaf. A soft, rotting rhizome needs attention: make sure it is not buried and remove decaying material.
Consider the transition
Some nursery plants are grown with leaves above water. Those leaves may be replaced with different-looking submerged foliage. Crypts can also shed leaves after a move. Keep conditions steady and avoid repeatedly uprooting a plant to check the roots.
Keep decay from accumulating
Remove loose, deteriorating leaves gently and continue normal maintenance. If there is substantial decay, test water quality and respond to the actual readings, particularly in an aquarium with animals.
Know when to change course
If no healthy growth develops and decline continues, revisit the plant’s suitability for your light, temperature, and planting method. A terrestrial plant kept underwater cannot be made aquatic by adding more fertilizer.
Further reading & references
AquariumFish.com: Cryptocoryne wendtii green ↗Tropica: Cryptocoryne wendtii green ↗Original practical guidance compiled for AquaticPlantSale.com. Plant response varies with species and aquarium conditions. Photography credits are listed on the credits page.