Fissidens can be a funny thing sometimes.. takes an age to get going, then all of a sudden it can grow at a silly pace, especially after a good trim. As others have said, it loves a nice yellow drop checker and good light, but will tolerate many conditions.
Are you sure you have Fissidens Fontanus? I've seen some eBay sellers listing other Fissidens species as Fontanus, and several of these don't enjoy life under water...