Cathode poisoning in nixies is caused by them being left on the same digit for a long time, usually days or months. Consistently running them on the same digit (first digit in a 12-hour clock, first digits on a frequency counter that doesn't blank unused digits) will also cause relative darkening of the adjacent cathodes, and occasionally some sputter will actually cause an internal short circuit which may or may not ultimately destroy the tube. Multiplexing should extend the life of the tubes. Running nixies at less than their rated current will increase their lifespan, and running them over-current for shorter periods will often rejuvenate them. Regards, Micah Mabelitini