Thank you Tom. I think so too.
Some of them appeared to be gasping at the surface, so I laid off the CO2 for a while and the gasping stopped. It then resumed with a only few of them. They started to die off. I would find them near the top with only a faint colour of red. The really bad ones I culled to stop any spreading of disease or infection. It seemed to work. I think I have 15 Cardinals now.
Thanks Andy. Atleast I'm not alone.
Concerning the background. I am considering a black background. In real life the rocky background doesn't look at all bad. I'll have a think.
I love Cardinal Tetras but I have had similar experiences with them. Some batches seem to be bullet proof and others seem to die almost as soon as you put them in the tank. I had 12 in a tank that where doing great for weeks. Added another 12 and overnight lost almost all of them, there was no spikes in the water chemistry or any obvious reason for losing them. Have also lost some to CO2 in the past as well.