My tank has been running for over two years and I have overcome many issues regarding plant growth and have finally got my stem plants growing at a satisfying rate. I have a 10 gallon tank with a Stingray fixture, EcoComplete substrate, and a paintball CO2 setup at around 30ppm. I am dosing Thrive ferts (one pump after water changes). Most of my plants seem to grow decently, as my purple cabomba grows like mad, my rotala grows at a steady pace, and my amazon sword is producing flowers and many plantlets. I have always had trouble with java moss, java fern, and dwarf sag. I have had the dwarf sag for over a year with no real visible growth. I am not sure if this is due to lighting.
My ph sits around 7.0 and as aformentioned, CO2 is around 30ppm (checked with drop checker and ppm chart). My GH and KH both are around 3. I do not know if this is related, but I have always had trouble keeping any shrimp species, though I doubt it is due to copper sensitivity as I use distilled water conditioned with Equilibrium and baking soda to achieve the desired KH and GH.
The reason you may be having issues with things like Java fern could be lighting. If a person puts a relatively strong light over a 10 gallon tank, you have basically turned that tank into a high light environment as there is 10 inches or less of water light has to travel through.
It is my understanding that Dwarf Sag is a plant that likes low to medium light. I'm not familiar with Stingray fixtures. Do they have a dimming option? If not, can you raise the fixture itself?
As for distilled water, it isn't the same as RO. Distilled just means that they filter out some stuff, but not all of it. https://www.epa.gov/sites/production...ttledwater.pdf
If you have concerns about issues with copper, there are test kits for that.
The following article is about growing a carpet without CO2, but you may find some useful bits of info in it. I'm posting it because of the Dwarf Sag's growing habits and the kind the light that it likes. https://www.advancedplantedtank.com/...ng-plants.html