i have played around with crypts for a couple years now... i noticed something.
if i moved them from soft water to hard water, they didnt melt, but if i moved them from hard water to soft water, all the leaves melted. i think some of the problems we have with plants melting stems from changes in osmotic pressure that causes the cells in the plants to explode. at the pet store my wife works at, they use RO water with very low total dissolved solids for their plant tank. the owner wont buy crypts because they have always melted as soon as he puts them in his tank.
i, on the other hand, have not had a single plant melt since i came out here to california. the water here is extremely hard with a high KH and PH(and i use tap). back in north carolina, with the soft acidic water, melting was pretty much expected with all plants prone to melting.