I have no idea why. I had a tiger barb and a swordtail for several months. Just put in 2 silver mollies a black fatail tetra and two red (???). Right froe start, one of the red w/black tail fish was not really moving around.

Within 2 weeks three of new ones were dead. Then, tiger barb died yesterday and swordtail today. Had gotten replacement for the siver mollie that died, and a algae sucker last night.

Had petsmart analyze the water. Everything looked good except soft water. I didn't know I needed a heater originally(didn't have one all this tim for the tiger barb and swordtail. I had know the water was too cool, but couldnt get the heater right away, so put some wrm water in the tank yesterday am. Got a heater last night.

Fish are lying on the bottom, almost not moving. When they swim, it looks like they are struggling to swim. The motion is not smooth, but jerky side to side.

Any ideas??

Why not to try a to consult a veterinarian . Ask them why that happened to your fish. And let them exam the water that your using.