Hi all
No experience with Fissidens but I think it can be a combination of some of the mentioned issues: here in the Mediterranean my tanks get to 30 degrees in summer quite easily and mosses do very bad (huge difference with winter, I do not heat my tanks or very lightly to avoid them being under 20 degrees... Those 10 degrees make a significant difference), shrimps are IMO the best moss cleaners (siphoning them will result in a disaster breaking their natural shape, at least for some species) and finally frequent trimming results in new and green growth.
I noticed that light can be important for some of them (Java/flame moss grow well nearly in the dark but a kind of spiky moss I have grows much better with good light), but not sure for Fissidens
Jordi