POLYFILTER ASAP, if it's copper or some other heavy metal it will change the color of the filter so you at least can confirm or eliminate heavy metals as a factor. IMO that's what the problem is. The bag on a polyfilter will tell you what is in your tank according to the color change on the filter, i.e. if it starts to turn green--copper. I would also get a ground probe just to eliminate the possibility of stray voltage as well.