Hi Patrick,
of course, if your plants died otherwise, the use of fertilizer sounds sensible to me.
… as long as your Paros’ eggs and larvae develop well.
I must also admit, the water from which I start isn’t as soft as yours, because our tap water is very hard (cond. around 1300 microS/cm). With peat I get my RO water to 45 microS/cm. Perhaps that’s some kind of limit for the growth of javafern etc.
Grettings,
Martin