Sometimes this just happens to java fern. I've had it happen several times over the years, both in tanks that are fertilized and not, it apparently just happens. The advantage of having multiple tanks is that you can always just reseed. I'm actually in the process of having it happen to a tank at the moment, it's a 490L tank with a low watt/litre ratio, lightly stocked. Where'd you get the java fern, Captain?