I know I don't have any in my crystal tank. Ph is usually 6.3ish and under. Ammonia (ammonium?) isn't achieved as quickly in a low ph environment, so that may have something to do with it?
Also, about the only thing they'll eat is snowflake, so not very much other food left over to infiltrate the substrate and cause rotting, which may also contribute to it..
I do see tiny black beard tufts under some moss, but not very big and the lower the ph is, the quicker it shrinks.