I just lost 3 tilapia yesterday, and one a couple days before that, out of about 25 in this tank. For the life of my I can't figure out why they died. They were all about 3 - 4" long and I've had them since mid-December. pH = 7, Ammonia, Nitrite and Nitrate levels all close to zero, temp - 72 F. and plenty of oxygen. A clue to the mystery, though, is that I've lost fish inexplicably in this tank before, and it is the only system we used PVC glue in. Could that be it? I've included a couple photos from my iPhone...hopefully they aren't as crazy big as they look now!
I know things are good now, or at I think that's what I read. Is that some hemorrhaging on the operculums, and on the forehead? Next time you have something like this, a good gill shot or two would be good, if they are really fresh dead.