Very cool. Thanks for sharing.
Hopefully moving the large dollars, shark, and upside down cat will fix the problem. I guess even in large tanks, if there is a lot of fish, there's bound to be interactions. I also think that boredom may play a role in aquaria.
I do not know what is the 'cyst' on the male silver dollar. However, I have seen similar spherical large 'cysts', at least once in a silver dollar, and perhaps once more, posted in forums. I searched in this forum for a picture, but could not find it. From my recollection of the post, and from your description it seems to be something that grows relatively slowly and does not appear to cause immediate problems to the fish (not acute). Instead it develops over several months, with apparently normal behavior of the fish. I would also like to know what causes them, and it would be nice to know if it is something that could be contagious. My guess is probably no, but what do I know? It would be worth considering a dissection/authopsy, particularly if the fish died. It also makes me wonder whether it is fluid-filled and can be drained.