I've never heard of this being a problem, but it could be:
Many people suggest that crypts grow best on wood/rocks where the roots are loose. A lot of us grow crypts in AS, Flourite, or other rock like substrates. Could the sand be too compact for the roots to grow properly?
That's what I was thinking. Since they are a root feeders, with the sand being too tight and no nutrients in the sand, this must be what the problem is. Some of my other crypt species in the tank are doing much better.
Also, what is the temperature fluctuation during a water change? It's likely that TDS doesn't change much, so I wouldn't be concerend about pH fluctuation. If there is a sharp change in temp of the water directly pointed at the crypt, that could be the culprit.
Tank temp is 77*f
New water going in is 77*f
My water pours in directly next to this plant
Maybe I should use some pH down before I add new water to the tank?