I had a similar problem with tetras... had them for a couple of years, then I had ordered 3 lemon tetras to add to the shoal and bam day by day i had at least 1 or 2 tetras dead every day for a week and a half until all 3 lemons were dead, then all 7 of my other tetra dropped dead... the weirdest thing. all the other fish were healthier than anything!