IME, java ferns are very tough plants. Their only issue is CO2/light.
I think in Bill's tank, which is non CO2, that the other plants are really dominating as growth ebb and flows with various species.
As they do, the scarce CO2 is used up........
In non CO2 planted tanks, you have a small pool of CO2 available.
This CO2 is very strongly competed for by the various species.
Some are much better than others.
Java fern does well typically in non CO2 planted tanks.
However, if you add a lot more species, many of which have reserves(like Crypts, those rhizomes are about 50% or more of the dry weight biomass- these serve as storage organs), those plants once established..............will use those reserves in the start of a growing season to get going and dominate/out compete based on total available surface area and biomass. the other thing is that when times are lean, later in the season.........then they have a back up and can flower, where the other plants are really hurting for CO2 or other nutrients.
The storage organs are mostly starch.......which is to say, reduced carbon........the ferns have little of this, often just sacrificing their leaves and the rhizome will sprout new leaves once the CO2 is higher.
So what to do?
Manage the growth, trim and keep the biomass consistent.
If you are a hands off gardener, then yes, much as DW's comment often state, she just watches the plants ebb and flow and phase in/out. I look at the system differently though and ask why that it is and then test and see what has been done research wise that seems plausible. So did she. However, The allelopathy argument does not hold much water.
I think CO2 is the without any doubt the biggest difference and we can easily test and see this in virtually all observations to date. Two big things suggest this:
1. When we add CO2, these types of issues go away and we can grow dense biomass, so with healthy biomass from all species, high production, should be not see evidence of allelopathy? It's just not there in any observations.
2. We may add activated carbon that removes specifically the allelopathic compounds. Again, same issue as ergo #1. No algae, no plant growth differences, help etc.
Why would Java fern do well without CO2, then when another plant starts growing well, the fern declines?
We can add nutrients, say like I suggest in the non CO2 article which is the water column based approach to non CO2, you can do that and know that the nutrients are not limiting strongly...........with aged sediments, this may.or may not be the case, also, Java ferns do not get any nutrients from the sediments do they?
So sediments are unknowns...........you cannot rule either way with them.
If you have soil/ADA AS or enriched sediment types, adding water column ferts in addition to them will help, we know it does not CAUSE ALGAE, WE KNOW THIS IS TRUE FOR NON CO2 AND CO2 ENRICHED SYSTEMS.
So............nutrients can be ruled out effectively.
What is left?
CO2 at a pretty low level, that's fairly hard to measure even with a CO2 meter.
CO2 is also the strongest limiting nutrient in any aquatic system except a few rich CO2 springs.
So this is the most likely suspect.