I've got an issue with my dart frog viv that I can't seem to solve. I have a small pond feature in the center of my 75 gallon tank fed by a waterfall running down the rock background. The pond is situated below grade with a moss covered area sloping down to it on three sides. This is the area that I use for feeding. No matter what type of moss I plant there, it turns black, especially the area closest to the water. I've tried regular sheet moss, Christmas moss, Java moss and it all turns black within a few weeks.
First, I need to figure out whether the black is fungus from excess moisture on the sloping area, or if it has something to do with vitamin/calcium residue from the feeding. Second, I need to figure out what to do about it. Any insights would be appreciated.