Didn't see that broomer already suggested that, it might be worth the moeny to have an outside company do a full analysis of the water . . You might check into a company called SGS they do some contract work for the company I work for... however it sounds like it might be to late, once again sorry for the loses.
BTW - what color if any did the poly change to.
I am not sure what hydrocarbons would be used in a bug bomb, but I would assume aromatic hydrocarbons such as xylene ( closely related to benzene ) . . this stuff is deadly to the bugs and people as well, so you can imagine what it would do to fish and corals when dissolved into water. If I had to guess I would say almost surely it was the bug bomb...the half life of hydrobarbons is, well it's a heck of a long time. I would try to find out what exactly was in the bomb. Either way, it dosen't look too good, sorry... the only thing is I can't figure out why it took so long to effect the fish. It could have been very stable and just took this long to fully break down.
-Josh-