When Nitrate gets above 20-30, it's time for a water change, whether you did one 5 days ago or 5 hours ago. However, the strips are often inaccurate. you would be much better to get liquid test kits for Ammonia, Nitrite and Nitrate.
If your plants are dying, then yes remove them, when they start decomposing, they will cause ammonia to rise, which in turn will up the nitrite, then nitrate, which you have indicated is already high. Water changes or plants are the only way to get nitrate out of the water.
Best of luck with the java ferns and moss. They are excellent plants to start with. I say any plant that won't commit suicide when I try to grow it, should grow for anyone, and my java moss is starting to take over, and the ferns are doing well.
If your plants are dying, then yes remove them, when they start decomposing, they will cause ammonia to rise, which in turn will up the nitrite, then nitrate, which you have indicated is already high. Water changes or plants are the only way to get nitrate out of the water.
Best of luck with the java ferns and moss. They are excellent plants to start with. I say any plant that won't commit suicide when I try to grow it, should grow for anyone, and my java moss is starting to take over, and the ferns are doing well.