Firstly, sorry to hear of your losses, HotStuffGal and understand how you may feel.
However, I agree with Guppygals' assumption about your tank is overloaded. I've tried and resisted the temptation to overload myself.
Overloading can help infection spread very quickly among your fishes, plus the strain/stress of having too many fishes together can exacerbate/increase the aggression amongst the more territorial fishes.
Solution. If you have a spare tank, use it. Give your fish some more swimming space. Definitely don't introduce these fishes into the other inhabited tanks, because if there is an infection you'll spread it.
Next, find the course of the problem. Ask yourself every question you can think of as to the cause, and keep an eye out for the signs that may have caused it. Usual things like regular fish health and water quality checks need to be maintained. Sometimes, (regular) water changes just won't do.
You can have anything from high ammonia, high nitrites, low aeration, unstable PH, or parasites as the cause of these deaths. I recommend you to get a test done of this tank, and your other tanks, just to be certain.
Lastly, post your findings in the forum. There are plenty of helpful, and experienced guys here who've gone through and learnt from similar 'mistakes' such as yours. You can also go to your local fish store who'll be able to see the afflicted fish/results obtained first-hand, and give you a handle on the situation.
Better still, you can survey the forum and find someone living near you, who you could go to, for further advice....if you so wish. There also some great fish clubs out there. Perhaps you can find something in the paper, Internet or phone book.
Good luck, HotStuffGal.