That sounds like a typical case of cryptocoryne rot to me. I haven't had that happen with only CO2 changes, but crypt rot can happen when the plants see a significant change in water conditions. It might be due to your changes in lighting and ferts.

I had the same thing happen when I transplanted two varieties of crypts. The leaves would just "melt away".

Give them time. If its crypt rot the plants will generate new growth in a week or so.