09-16-2010, 04:45 PM
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I read back through this thread and didn't see numbers of specific fish so I can only provide general suggestions.
First, I warned against cardinals in this hard a water. While I can't say it was the water hardness that took them, it probably didn't help. These are very delicate fish internally. And a fish that is "outside" its preferred water is going to be under stress. That means when something else happens, as for instance the cycling issue, it is doubly hit. Then there are the Tiger Barbs.
As redchigh correctly said, these are not community fish. In a 75g they can work, but only if they are in a large group (at least 8, preferably more) and depending upon their tankmates. I see no indication as to how many there are, so the behaviour you describe could be aggression between rivals. If you read our profile of this fish, you will find the reasons why they need to be in larger groups.
Cardinals by comparison are very sedate fish, and Tigers in the same tank could well make the cardinals jumpy, which again causes stress and this weakens the fish to everything else.
One thing that is essential to grasp when considering fish species for a community aquarium is that not all small fish will live together and for many different reasons. Water requirements is one important aspect that many overlook; another is the inherent behaviour of the fish. And environmental factors can often bring out traits that might otherwise never be seen. A few weeks back I posted a link to a scientific study that supports what many of us have been saying for a long time about the reasons for keeping shoaling fish in groups, and large groups--reducing aggression. When even normally peaceful shoaling fish are maintained in a pair or just a few in an aquarium they can exhibit aggression unknown in larger groups.
The fish profiles can be accessed from the second tab from the left in the blue bar at the top.