I am pretty sure that the plants in the picture had a peat soil mix, roughly 50:50. Also I had sprinkled some broken up egg shells in the mix also, something like 1/2 teaspoon of fragments small enough to go through a rice strainer in about 24 cubic inches of soil, to keep it from getting too acidic. I have never seen the species melt underwater, and it seems to be more tolerant of varying conditions than a lot of other crypts. I don't think it is especially high light, but it may not be quite as tolerant of low light as others. As crypts go, it is probably pretty high-light, similar to C. ciliata.
I don't always have the time to take the best care of my submersed plants, and once, to my surprise, a plant of C. lingua along with a plant of C. chordata survived about 4 months in a pea-soup green water tank with a few guppies and no CO2. The pH was probably around 8 to 9 for four months. when I finally drained out all the green water, there the two plants were. They hadn't grown, but they still looked reasonably healthy. Crypts do seem to have a tolerance for low photosynthetic rates, and that is probably why they survived under those conditions. A high pH, low CO2 tank is sort of like being in very dim light for them. They can take that.
I have one C. lingua plant now, growing nicely in a tank where some of the other crypts are showing a few problems, perhaps iron deficiency along with something else; time will tell*. The lingua , however, looks great. The lingua is in peat-soil. The plants with problems are in gravel with only a small amount of soil and no peat at the bottom.
I did fertilize the water column with small additions of nitrate and phosphate for the plants in the picture. I didn't have a test kit then, but I am quite sure that nitrate levels dropped to unmeasurable values between additions, which were not large. The water was slightly tea-colored, but not a lot.
I am going to do a separate post with a picture on the problems of these plants. First I have to get rid of a big mat of Rotala sp. Nanjenshan that is growing at the surface and cutting out the light.