This looks to me like mite damage. Do you have magnifying glass around that you can use for a closer inspection? I try to keep all fo the plants that I produce clean and bug-free, but using regular houseplants in a riparium can often introduce trouble.
I looked through them and don't really see any mites. Having said that this is the only plant that is showing any signs of stress, so I dont really think it came with the mites, I think if anything they came from the water lettuce that the fish eventually ate up.
how do I treat it for mites anyways? I would rather be safe than sorry, plus I really like this little tree and don't want to lose it.
I actually cut off the burnt leaves from the fern and it seems to be doing well. I also repositioned the planters and rafts so that there was a more balanced feel to the tank. I had my bf send me a photo to post, but ill take some more later.