I do 3 week filter changes on the tank and don't change both at the same time, I have two EMP 400. Can't think of anything that could have led to contamination right off, but it is possible. No introduction of new plants, gravel, fish, or anything I haven't been doing for 1 year. I clean all food particles out as soon as they are done eating, they are fed once per day with alternating krill and frozen shrimp which is peeled. From what I am reading this seems to be bacterial of some sort. Is that the wrong direction? If it were water it would be affecting all fish all the time I would think.