I know in my own experience, Crypts can melt going from lower to higher light, though I've never seen actual melting with other plants I have seen them react - for instance, yesterday I moved some Bacopa australis from low-med light into my high light tank, and it's looking a little... droopy. Plants that do this seem to perk up in a few days in my tank, but there is often a definite reaction to the change. I've got a book that has an interesting explanation for Crypts melting, see what you think:
The following is a quote from Ines Scheurmann's 'The New Aquarium Handbook', ISBN 0-8120-3682-4, published by Barrons in 1985 - obviously it's not really a 'new' book any more!
"...Cryptocoryne rot. It manifests itself first by holes in the leaves and around their edges that look as though they had been nibbled by snails or fish. Then, within a few days, an entire plant or Cryptocoryne grouping may collapse and rot. [...] Cryptocoryne rot is caused by an excess of nitrates. Our tap water contains much more nitrates than the tropical waters that are the natural habitat for Cryptocoryne. Leftover food particles, decaying pieces of plants, and fish excreta continually add nitrates to the aquarium water. Plants that naturally grow in water high in ntirates can break them down into ammonium, which is a plant nutrient. Cryptocoyne and other tropical plants, however, are used to living in water with plenty of ammonium and thereofre have not developed the ability to break down nitrates. They take up nitrates just as they do other plant nutrients, but store them with other unusable waste products in their cells. When there is a sudden major change in the aquarium environment, these plants cannot tolerate the shock. (Such a sudden change occurs, for example, when the aquarium hobbyist finally changes the water - a chore long overdue; or after a long time, adds fertilizer; or replaces and old, worn-out fluorescent tube.) To overcome the shock of the sudden change, the plants reactivate nutrient substances that awere stored as reserves and thereby release stored nitrates at the same time. the tixic nitrogen compounds then formed kill the plants."
Interesting theory, not sure if there's any merit to it at all, just thought I'd throw it out there as food for thought
