I don't like PTNuke. Well, I thought it might be responsible for some blackish gunk build up inside my last block which was kinda sorta corroborated on another forum, but I can't be sure and couldn't prove anyway. It would probably be more common knowledge and could have been some other contaminant(s). But just been just using a kill coil since and try to change my water every six months at the most and so far no visible problems. I should actually open up my new block soon though which has been running almost a year for a look-see though.