I'm guessing that one of the variables with water
ferns and probably any plants, is how much light they
are getting. And how long they are getting it.
In our village, the municipality does all of us a
favor by taking out the sulfer in the water (leaving
the memorable rotton egg smell) and the iron which
stained shirts and laundry. Their village sized
version of a basement water softener exchanges a
significant quantity of sodium for some of the calcium
and magnesium in the water. Some feel that is a favor.
I just couldn't grow water sprite in the tap water. A
friend down the street, with more light intensity and
a much longer light day, indeed sometimes 24/7, grew
magnificent water sprite.
I'm not explaining it properly, I'm sure, but it seems
that his better lit plants were able to generate the
energy to overcome the (osmotic?) problems caused by
the sodium and fairly hard water. That probably
explains why I'm now more serious about cutting the
tap water with R.O. water or even rebuilding the R.O.
water and slipping more, and more effective, over head
lights into the fishroom (and dropped them closer to
the tanks). Still not growing great water sprite
except in a lucky corner, but the low light "Killie
plants", such as Java moss, Java fern and some
Cryptocoryns, which minimally may only need 1/2 to 1/4
the light intensity of the water sprite, are certainly
doing better.
All the best!
Scott