The leaves will be wilted looking around the edges, and not as green (either yellow, or not their usual vibrant green). You need more potassium and nitrate.
If you need any (I know you're breeding) there's a post in the equipment classifieds.
If not, I'd maybe just trim the melting\ugly leaves for a while. Unless it has bad chlorosis, it will be remedied by dosing within days. Perhaps just pull the ferns in the meanwhile, fortunately they're a cheap and readily available plant.
What's happening is your plants aren't able to produce chlorophyll due to the lack of nutrients. I'm not sure if reducing the photo periods will help the situation, as the root issue is nutrients.
Brian are the tips just clearish. If so this is the way it naturally grows. The cells at the growing tip seem to fill in with chorophyll after a while. I posed this question to Tim Burton of Aquaflora Nurseries, and he said this is the way this fern can grow.
I thought my fern tips were dissolving too when I asked Tim about the phenomenon. I hope this resolves your issue. All my varieties of Java fern grow with this behaviour. The only variant where it is least prevalent is the needle leaf.
If parts of the leaves become mottled/muddied with brown areas, this is usually a result of low nitrates!