Sorry to hear of some fish issues. If you get a chance, can you post a picture for us? My guess based on the description of the tetra is fin/tail rot? there are numerous medications that are made for this. i do not know which one works the best, as I haven't personally experienced it. I'm sure someone has dealt with this before, and will chime in shortly. As for loss of appetite, can you share your water parameters? I wonder if something is out-of-whack with the water...... if you can post results, we could steer you in the right direction.