Unexplained Sudden Deaths

Hi

I have 27 Cardinal Tetra's and 16 Harlequins which I have had for about 5 months. Never lost one and never had any problems. But two days ago I got up to find one of the Harley's dead on the bottom. Today when I got up they all looked fine, no erratic swimming, signs of illness or other unusual behaviour but while I was watching them, one of the Cardinals literally dropped dead and sank to the bottom

Water parameters are spot on and all the fish look really healthy. Just bad luck and coincidence or could it be something more sinister?

Sorry, WR. The only thing that occurred to me was 'one of those things' which isn't much help to you.

In your situation, I would just keep an eye on the fish for any signs of something wrong. Without any physical symptoms to go by it is impossible to say for definite if there is a disease, for example.
It could just be co-incidence that both fish died so close together. A lot of fish these days tend to have weak members simply because of mass breeding on fish farms. A private breeder would use only healthy fish to breed from, and would cull any sickly fry. Fish farms do neither.