Originally Posted by rchbn
...and it's getting expensive to replace them and then they aren't the same size etc.....
So this isn't the first time? How many fish have you lost over what time period? Obviously, there could be hundreds of different reasons your fish are dying, but maybe we can home in on some possible reasons.
Are the fish that are dying the newest ones put in? If so, how long do they last? And are you quarantining the fish before putting them in the main tank?
[Edit: Looking back at your previous post at the end of March... did you ever positively cycle the tank? Looks like back then you were putting fish into an uncycled tank. That'll kill 'em every time.]