i don't know anything about fissidens specifically, but i do know that a good number of aquatic/semi-aquatic plants have different leaves depending on whether they're emersed or submersed... could it be the same way with the fissidens?
EDIT: assuming your 90P is currently submersed. i read it that way.
Right, most definitely they report different growth. I have 2 90ps, the emersed setup in this thread and a submersed 90p that I've had going for quite some time now. In the submersed tank, I have two swatches of fissidens, and up until now, I thought them both fontanus. However, although they are right next to each other on the substrate, one exhibits tiny, rather dainty structure while the other is much larger and also more leggy. The difference is quite distinct.
Similarly. In this tank, the emersed one, I am pretty surprised how different mini pellia can look. Close to high light it's a completely different plant. Actually, if one didn't know better, it looks like I actually have 3 different types of emersed pelia, but I know them all to be the same species.