Lol. I don't keep aggressive fish. It seems to be any fish added in the last year. It came to a head for me when I lost about $2000 of fish within 2 weeks of transferring them into this tank. Just for fun, I treated with pranziquel this morning.
So, is this tank you are transferring them into a new tank? Perhaps its leaching something in the water somehow...
Thats about the only thing I could think of, if the only new variable to the equation is the physical tank, it would have to either be something with the tank or crazy coincidence.