It definitely isn't ich....only other thing I can think of is hole in the head. I recently lost a lot of lemon tetras from something that would eat away part of their face gills and body, like lesions. Could it have been hole in the head all along? here are some videos of the tetras and another of the angelfish. If not what in the world is it and what should I use? The lemon tetras are replaceable but the angel is the star of the show.
http://www.youtube.com/watch?v=ty8H4cxsL0Y&feature=plcp
http://www.youtube.com/watch?v=ouaN85tkBBQ&feature=plcp
http://www.youtube.com/watch?v=ty8H4cxsL0Y&feature=plcp
http://www.youtube.com/watch?v=ouaN85tkBBQ&feature=plcp