Kate wrote:

has anyone ever heard of fish being poisoned by fertilizer tabs? How about
hygro of swords being harmful to lemon tetras? I'm just trying to eliminate
my plants as the sudden cause of death for all three of my two year old
lemon tetras.

It sounds unlikely. I am unfamiliar with the lifespan of lemon tetras, but
current thinking is that cardinal tetras are essentially annual fish in the
wild: only a few percent outlive the season of their birth.