Thanks for the details! Very nice set-up.
To my knowledge, fissidens grows rather slowly so it probably wont overtake your tank.
Also, it seems to really like high lighting and nutrients
I am not so concerned about the rate of growth, just if it will attach easily to whatever it wants to. Java moss grows slowly but I hate it because it will attach to anything and everything. In a tank where I don't want it, it's worse than algae. I actually have a bit of Java moss in this tank that was accidentally introduced and I am almost rid of it but I have to scrape the rocks with a razor to make sure it doesn't come back.
I do have more light than would ever be reasonable if I lowered the light down to the rim. 98 watts of T5HO with decent reflectors. I can dose as much as I want. I will likely wait for the foreground to grow in before I try.
That said, although I lost some of my belem (which was growing terribly slow before I switched things up), it's growth has noticeably improved in the last few days. I am guessing it was my light because I did have the same CO2 and filter setup for at least a month or two and it didn't seem to do a whole lot over no co2 and HOB filters. I lowered my light 2 inches which may be too much, I will see on Sunday or Monday when I am home. I don't know why but I like to make lighting changes when I am not going to be home. I guess it gives me enough time to see my mistakes lol.