Hi, Jordy – and belated best Wishes!!
Even if you will change it later – don’t plant the Microsorum into the ground, it doesn’t like a covered rhizome.
And be careful with adding stones – I already had to change a complete tank, because the stones I had added increased the conductivity. (Some wood also does!!)
Why did you add 1 third normal tap water? The values of the rain water sounded perfect …..