Well, since it is a 20 gallon, there are two pissible causes of the Java fern problems. The first is the lack of nutrients because it can be a nutrient sponge. I would dose the Flourish at the regular dose twice a week to see if the moss responds. Maybe even consider getting some NO3 to dose.
The other problem is 0.75 watts/gallon is extremely low for a 20 gallon and is equivelant to nearly no light at all. GO HERE
and input your tank dimensions. I know it sounds odd but I have 33 watts over my 10 gallon tanks and have very little problems with algae without CO2. I do dose Excel now but even befor eI got it the algae was minimul and was much worse when I had DIY CO2 and couldn't keep stable levels. Anything that comes out on that chart below low light may not be enough to grow much of anything
Give the moss some nutrients and possibly consider getting some NO3 for it because it sounds like your levels could be too low to benefit the moss. If that doesn't work, then the moss needs more light. And yes, Anubias can be grown in light levels that you can barely see but the growth will also be barely seen. I have mine under 3.3 w/g in 10 gallon tanks and I can actually see the leaves growth daily. I can only imagine their growth rates in higher light. I could not grow java moss in my 10 gallon tanks with 1.5 w/g, it suffered from lack of light and never grew fast enough to produce more than was rotting away.
BTW, there is a difference between fish waste nutrients and dosed nutrients. Dosed nutrients are readily available to the plants and they can be used as is by the plants. Broken down nutrients will test high sometimes but you can still see nutrient deficiancies in plants because it is not all readily used by the plants.
As far as overdosing, here is what I dose a week:
0.9-1.1 ppm Iron
Even if my plants use none of it, I can have a maximum buildup of double that amount because I do 50%+ water changes weekly.