It's been an ongoing issue. Most plants end up getting a trim before it's to bad, but the ferns are a slow grower and have just gotten worse. And have always struggled with stem plants.
Not tried remineralized before.
Kh is 4 and GH is 10
If it was me, I’d start now with co2.
If co2 were correct, all plants would be absolutely gorgeous for about a month after being freshly planted. (Given a meticulous water change routine at startup).
We will need a full tank shot and run down of your entire filtration and co2 implementation.
Going to take at least a week and probably a month to get sorted then a month or three to get the proper results you want.
Keep everything consistent outside of co2.
Without any additional data, my first step would be this:
Leave co2 injection rate as is. Turn it on 1 hour before lights on and off 1 hour before lights off.
If that is not your current set up, you need to watch for 1-2 days. Then slowly turn up the needle wheel on your injector a micro turn. And watch the plant and fish response closely during that day.
If you overshot (the fish will look lethargic maybe at the top and it’s possible if you do this while you are not home, they will be dead- so you must be very careful).
If the fish are healthy and the plants perk up we are great.
If the fish don’t look good and your plants don’t look any better, we turn to overhauling the distribution method of your water.