Well, java fern is pretty resilient. By the looks of yours, even though the leaves are browning, the roots still look very healthy. They are long and have those bristly hairs which means they are in good condition. Leave some of the browning leaves for a few weeks and see if they sprout seedlings. After most of mine darken, they start spawning seedlings like crazy. I've got too much java fern already.
Someone else may have to chime in about excessive water changes. If your levels are all near zero, there's no reason to change. When nitrate starts building up, its part of their diet. Maybe try backing off on water changes & hold your nitrates around 30ppm.... see if the plants respond. You said you had water problems in the past...since plants feed off that excess ammonia and nitrate, they may be starving. Instead of purchasing stuff, I'd just back off on the water changes. 25%/week is healthy for a normal tank. My tanks with plants get 25% every 2 weeks.
Good luck.