All I read said that Crypts need a rich substrate. Must have been the build up of mulm. Very interesting! People whom have hard water though seem to have no problems with plants.My first planted tank used the pea gravel substrate the tank originally had, though it had a decent buildup of mulm from years of fish inhabitation. Cryptocoryne wendtii did decently well, but jungle Vallisneria by and far dominated the tank (though my hard water may have been a factor in this). As for stems, Rotala rotundifolia did pretty well.
This is going to be my breeding tank for threadfin rainbows, thus I don't plan to put a lot of plants in it.You could use a finer, plant oriented substrate, perhaps an inch or so deep, and cap it with gravel. That way you have the look of gravel, but the benefits of a soil sort of substrate.
In a gravel only substrate be very cautious about substrate tablets. The water movement through the gravel can dissolve the tablets pretty fast.
I know of other local keepers who've had success with other crypt species like C. balansae in similar setups, while I didn't. However their tanks had been set up for substantially longer than mine (decades).All I read said that Crypts need a rich substrate. Must have been the build up of mulm. Very interesting! People whom have hard water though seem to have no problems with plants.