Ahhh, Australia...
How do you keep it hydrated there?
I have several staghorn ferns, and I put some wet moss around a piece of wood (no salts in the wood) and then I used copper wire to secure the plant to the whole thing. I kept it in the shade, which, in our zone 8b in south Alabama USA is quite humid. I do not fertilize it at all. I do spritz the leaves, which do NOT like to have dust on them, and neither do they want you to remove the fuzz. A healthy leaf is a fuzzy leaf.

What sort of normal humidity do your ferns have? And what size is this plant you are having trouble with?

Probably putting some compost tea in a tub/container and immersing the plant in it will help to remove all the overdose of chemicals for a fresh start. Then let it drain.

Let me see if I can find a shot of my young plants, some of which even started new babies within the moss before they had fully attached to the wood board.

In the summer, I hang my young plants by chains from the tree limbs outdoors, in the shade of course.

One thing I discovered was SCALE on some of them, when I had a scheffelera near by. I finally got rid of that scale magnet, and cleaned up the scale on my staghorns, and now they are all doing nicely. Good luck with your young plant.

{{gwi:616734}}
{{gwi:616735}}

and here are the babies and a good view of the moss. No soil used at all. Keep the moss damp, not soggy.
{{gwi:616736}}