Doomed to Crash???

[dkcrx]

could be something like internal parasites like heart worms or gill flukes, seems to be a problem that will start by killing off the weekest first and continue, or stop all together if your other fish are in good health. I had this happen 6 or 7 years ago and lost a few one day and buy the time I figured it out I lost nearly all my prise fish "near $1000 worth" had to do a little disecting to find out.