Java fern as easy as it is following description also is one of the slowest around. Especialy in a low energy invironment. That's also what makes this plant very hard to read to get an indication. If yuo have other plants growing well, than likely there aint a defficiency. Well maybe a defficiency in patience.. If you realy like a big lush green java fern in the shortest periode possible you definitively need to add CO². In a low energy chances are you need a few years. Java fern leaves don't live for ever, they have a life cycle.
Brown spots could indicate it's at the end of it, some just wither away others develop new plantlets at it's underside.. Size says nothing about it's age. Brown spots never cure, so even if you change fert regime it'll take to long for your java to show that, it will tell you nothing. Rather focus on other faster growing plants as indicator..
about 2 years ago i setup my low energy and did put in about 5 different Microsorum sp. (Orange, taiwan, needle, philipine, regular) all where submersed form grown in a CO² invironment. Some where rather big.. But 80% from the mature leaves died with in a few months from the sudden transplant chock and i had to start over from scractch. From the Orange i only have some tiny young new plantlest left over, the needle leaf i can't seem to find back. The only sp. still obviously present and grwoing are the Tawian, the regular and the philipine.. The rest remain a mystery and no idea if i ever see them again. But i have time. Tho for a low energy i think the Orange was a waste of money.. In a low energy this likely never grow into an orange and stay indistinguishable from any other regular java fern.
So also when you buy it from the LFS it likely is a emersed grown plant from the nursery, it will likely suffer a transplant shock going submersed. There is no garanty all adult leaves will survive this. If they don't they can die as slow as they grow. 🙂