In one the leaves are tiny and immaculate. In the other they are leggy and rather large. This has me thinking they are different species.
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.