Something is eating out the solid green sections between the veins of my java fern leaves, leaving only the bare veins with open spaces where the green once was. These sections are either being eaten or are simply rotting away, leaving the veins intact.

I currently have in my 20-gallon several red cherry shrimp, some blue shrimp which have bred, a variety of snails (mostly small), and seven white cloud minnows.

When I change the water, I add Flourish Comprehensive and API root tabs containing iron but no copper.

Recently, a nearly solid sheet of duckweed has covered the surface of my tank, and I have gotten rid of most of it. This skeletal outline problem was occurring before the duckweed was ever in my tank.

This doesn't happen to all of the java fern leaves, nor does it happen constantly. It seems to happen off and on, and when it does, several leaves are affected at once. In between these bouts, I have nice, green, full and healthy leaves for extended periods of time.

What could be causing this? Lighting too low, perhaps? What nutrients could be lacking? The plants do get iron as well as whatever else is in the ferts I use. This condition seems to escalate after I have had vacation food in the tank while I've been away.

I should add that only the java ferns are affected. Nothing wrong with my crypts, amazon sword or anubias, or any of the floating plants.